meta: mention experimental nature of UIKitless feature

[armcknight]

There is some confusion over the availability of the UIKitless feature, like in #3460

Point out this is still an experimental feature not yet compatible with SPM.

[github-actions]

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🚫	Please consider adding a changelog entry for the next release.

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## Unreleased

- mention experimental nature of UIKitless feature ([#3470](https://github.com/getsentry/sentry-cocoa/pull/3470))

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[philipphofmann]

Thanks, @armcknight 👍