Increase z-index of Select Dropdown element

[sierikov]

Increased z-index of select's dropdown element. Closes #4447. Tested manually.

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

A picture of a cute animal (not mandatory but encouraged)

[github-actions]

Hello there sierikov! 👋

Thank you and congrats 🎉 for opening your first PR on this project! ✨ 💖

We will try to review it soon!

[germanosin]

Me and @Haarolean no longer support this repo, don't you mind to post this to https://github.com/kafbat/kafka-ui