Tweak header hover effects

[HiDeoo]

• Closes Hover effects shouldn't decrease contrast/opacity in header #3263

As suggested in #3263 (comment), this PR updates the social icons hover effect to use the same hover effect we use in other parts of Starlight rather than relying on an opacity decrease. Turns out that this was something I personally changed in the Rapide theme a while back so definitely happy to have the same effect as default in Starlight.

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The second change is updating the select (theme and language switcher) color and hover colors to use a contrast increase rather than a decrease. This matches what we currently do for example in the search bar or mobile table of contents.

As this change was not explicitly requested in the linked comment above, this is done in a separate commit so we can easily revert it if needed.

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Remaining tasks

• Add changeset with instructions on how to preserve old behavior (I will only do this after confirming all the changes we want to ship)

[changeset-bot]

🦋 Changeset detected

Latest commit: 9ac528f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Minor

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[delucis]

I think this change makes sense. It is subtly “breaking” but I think that standardising that increase in contrast everywhere is logical — thanks for the implementation!

I can take care of writing the changeset, given you’re hard at work on v6 stuff!

[delucis]

Added the changeset, so this should be ready whenever we want to release the next minor.