These are approximately equivalent.

2 Skill proficiencies are more useful than a language overall (both in my experience and according to the rulebook in so far as you can train languages, but not skills, with the Downtime rules in the Player's Handbook), so you have to be careful, but losing your choice of a cantrip for one of the least useful ones (in my experience) should account for this discrepancy.

This can also be seen by comparing the Half-Elf and High Elf racial traits side by side (as these official races are roughly balanced with each other).

">" is slightly better than; ">>" is noticeably better than; ">>>" is much better than

\begin{array}{rcl} \bf\text{High Elf} & & \bf\text{Half-Elf} \\ \hline \text{+2/+1 ASI} & \text{<<} & \text{+2/+1/+1 ASI}\\ \hline \text{Fey Ancestry} & \text{=} & \text{Fey Ancestry} \\ \hline \text{Darkvision} & \text{=} & \text{Darkvision}\\ \hline \text{Extra Language} & \text{=} & \text{Included}\\ \hline \text{Keen Senses} & \text{≈} & \text{1/2 Skill Versatility}\\ \hline \text{Cantrip} & \text{>} & \text{1/2 Skill Versatility}\\ \hline \text{Trance} & \text{>} & \text{none}\\ \hline \text{Elf Weapon Training} & \text{≈} & \text{none}\\ \hline \end{array}

I consider Keen Senses and "skill of your choice" to be roughly equivalent since Perception is one of the more useful skills available from experience and WoTC Adventure Module analysis. Elf Weapon Training is a very minuscule benefit.

Now let's take your Sun Elf Homebrew and compare it in the same manner.

\begin{array}{rcl} \bf\text{Sun Elf} & & \bf\text{Half-Elf} \\ \hline \text{+2/+1 ASI} & \text{<<} & \text{+2/+1/+1 ASI}\\ \hline \text{Fey Ancestry} & \text{=} & \text{Fey Ancestry} \\ \hline \text{none} & \text{<} & \text{Darkvision}\\ \hline \text{none} & \text{≈} & \text{+1 Language}\\ \hline \text{Noble Heritage} & \text{<} & \text{Skill Versatility}\\ \hline \text{knows } light & \text{>} & \text{none}\\ \hline \text{Trance} & \text{>} & \text{none}\\ \hline \text{Elf Weapon Training} & \text{≈} & \text{none}\\ \hline \text{Celestial Resistance} & \text{>>} & \text{none}\\ \hline \end{array}

This hinges on the fact that Necrotic and Radiant are rarer damage types in your setting and that History and Persuasion are not used more often in your campaign. In a setting with heavy intrigue and/or many creatures that deal Necrotic or Radiant damage, this race may be unbalanced as Celestial Resistance is pushed to much better than status and/or Noble Heritage is roughly equivalent to Skill Versatility (I've not done any analysis on the proportion of campaign-styles where this is relevant like the WoTC balance team likely did, so I can't say for sure if this is balanced overall, but it seems to be).

It's only slightly more powerful than normal, but would make existing spellcasters less special.

To begin with, you're giving every character something else on top of their race, class and background abilities. This is by definition going to make them more powerful or versatile than standard, if only slightly.

How much more powerful? Not hugely. Cantrips are already available to a 1st-level party and elves already get one wizard cantrip for free, so it's not entirely beyond the range of capabilities of a 1st-level character.

In terms of effectiveness, assuming the players will naturally choose optimal spells:

• The fighter normally deals 1d8 with a longbow or 1d10 with a heavy crossbow. A fighter who learns fire bolt will deal 1d10 damage. At higher levels they will deal more damage, but they would have had multiple attacks with weapons anyway.
• The fighter who takes true strike will use it to gain advantage at the start of every combat encounter where they party has the element of surprise to prepare buff spells. However, in such a situation they may already be ruled to have advantage from having surprise.
• Any character might take spare the dying, which will prevent wounded characters from dying. This seems entirely reasonable to me.
• More characters will find it easier to deal elemental type damage against creatures who are resistant to weapons.
• More characters will be able to inflict tactical penalties like shocking grasp preventing reactions or thorn whip's ability to pull creatures closer.

All in all, this isn't massively overpowered (but beware of of other sourcebooks, which may add spells intended for a wizard that are overpowered in the hands of a fighter).

The main issue here is that you're handing out other classes' abilities, which makes spellcasters less special. A wizard at 1st level has only two 1st level spells per day and his main utility after that consists of three cantrips. If you give everyone a cantrip, you're breaking the separation of party roles and making the party's arcane spellcasters less important.