[EuiButtonDisplay] Fix missing text-align CSS

[cee-chen]

Summary

Note

This doesn't fundamentally affect any actual visuals, because flex CSS was already handling centering button content for us. But it's nice not to have buggy CSS-in-JS in our codebase 😅

This was missed during EuiButton's original Emotion conversion. Just for funsies, I'm going to dive into what was causing the bug:

• Our logical*Style utilities output objects of CSS key/values, e.g. { color: red }

• Our logical*CSS utilities output strings of CSS, e.g. 'color: red;'

• Emotion CSS that uses the css`` template literal can parse both.

• Regular template literals (``), like the one being used in euiButtonBaseCSS (to avoid generating an extra emotion label, presumably), cannot parse objects, only strings. So the previous code was generating this:
  
  Huge thank you/shout out to @tsullivan for logging and noticing this bug!

• Changing our internal EUI code to use the string output instead of object output solves the issue. We now have text-align: center in our CSS again, where it was previously missing.
  

• Bonus: another approach I could have tried to resolve this was potentially using the css`` template literal and renaming the util to _euiButtonBaseCSS. Emotion generally seems to respect and not add extra classNames to underscore-prefixed utilities, although this is not always the case.

QA

• Go to https://eui.elastic.co/pr_8057/#/navigation/button and confirm text-align: center is now present on our buttons, where it is not on prod

General checklist

• Browser QA
  • Checked in Chrome, Safari, Edge, and Firefox
• Docs site QA - N/A
• Code quality checklist - N/A, styles only
• Release checklist
  • [ ] A changelog entry exists and is marked appropriately. I'm opting to skip a changelog for this since it's an internal-only component / tech debt and doesn't affect any consumers or end-users.
• Designer checklist - N/A

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: f97f4c2

[cee-chen]

@elastic/eui-team Re-ping for any review :)

[tkajtoch]

Tested on PR environment; the change looks great!

I kinda hate how easy would be to make that mistake again with these function names...

[cee-chen]

Very fair - we could try to add a lint rule for it 🤔 but most likely we just need strong visual regression testing?