LUCENE-10440: Reduce visibility of TaxonomyFacets and FloatTaxonomyFacets

[gsmiller]

Description

These two classes are really implementation details, meant to hold common logic for our faceting implementations, but they are public and could be extended by users. It would be nice to reduce visibility to shrink our API surface area.

Solution

Make pkg-private. Also reduce visibility of protected methods/fields to pkg-private as a little extra cleanup. Note that I will mark these as @Deprecated on the 9x branch to provide advanced notice to any users that might be extending these.

Tests

No new testing needed.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Lucene maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.