Make Amsterdam an opt-in theme (#212787)

## Summary

This PR updates `DEFAULT_THEME_TAGS` used to determine what theme tags
are bundled in Kibana by default to only include the Borealis theme,
specifically `borealislight` and `borealisdark` theme tags. This change
is expected to decrease bundle sizes significantly and get back to
bundling a single theme, not two (4 → 2 theme tags).

Now that Serverless, `9.0`, and `main` all run with Borealis, there's no
risk in removing Amsterdam from the bundle and decreasing Kibana bundle
sizes.

We need to keep the feature flag in code for the time being to easily
test future Borealis iterations.

Amsterdam will still be available as an opt-in theme and is meant to be
used locally when testing changes to be backported to 8.x versions that
use Amsterdam. To do so, Kibana needs to be started/built with
`KBN_OPTIMIZER_THEMES` environment variable set and the feature flag
overridden in `kibana.dev.yml`.

```yml
# config/kibana.dev.yml
feature_flags.overrides.coreRendering.defaultThemeName: amsterdam
```

```shell
# Run dev server with both borealis and Amsterdam theme tags
KBN_OPTIMIZER_THEMES="borealislight,borealisdark,v8light,v8dark" yarn start
```

### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

packages/kbn-optimizer/src/optimizer/optimizer_cache_key.test.ts

@@ -91,8 +91,6 @@ describe('getOptimizerCacheKey()', () => {
           "reactVersion": "17",
           "repoRoot": <absolute path>,
           "themeTags": Array [
-            "v8light",
-            "v8dark",
             "borealislight",
             "borealisdark",
           ],