fix(OnyxFormElementV2): use correct label gap and input padding (#5066)

Relates to #4918

Update OnyxFormElementV2 to:
- use correct gap between label + body in compact density
- use correct inline padding for input when leading/trailing icons exist
- add missing deprecation docs for unstable components

## Checklist

- [x] The added / edited code has been documented with
[JSDoc](https://jsdoc.app/about-getting-started)
- [x] If a new component is added, at least one [Playwright screenshot
test](https://github.com/SchwarzIT/onyx/actions/workflows/playwright-screenshots.yml)
or functional test is added
- [x] A changeset is added with `npx changeset add` if your changes
should be released as npm package (because they affect the library
usage)
- [x] I have performed a self review of my code ("Files changed" tab in
the pull request)

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: larsrickert

.changeset/whole-cycles-carry.md

@@ -0,0 +1,8 @@
+---
+"sit-onyx": patch
+---
+
+fix(OnyxFormElementV2): fix styles
+
+- use correct label gap when skeleton is in compact density
+- use correct inline padding for input when leading/trailing icons exist