SOLR-15111 Use JDK8 Base64 instead of own implementation

[asalamon74]

Original lucene-solr pull request: apache/lucene-solr#2252 I removed the lucene part of it.

Description

JDK8 has a builtin Base64 encoder and decoder, there is no need to use own implementation for this.

Solution

Eliminate own implementation.

Tests

Unit tests

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 Solr 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.
• I have added documentation for the Reference Guide

[janhoy]

Looks mostly straight forward. But I think a few places can be simplified by calling encodeToString(<bytebuffer>.array()) rather than new String(encode(<bytebuffer>).array, <charset>), see comment.

[asalamon74]

Rebased on top of the current main. @janhoy can you please take a look at it.

[janhoy]

I have no further comments. If tests pass I’m happy :)

[janhoy]

Please add a CHANGES.txt entry for version 8.10, and I’ll commit.

Have you run the entire test suite?

[asalamon74]

@janhoy Added the CHANGES.txt entry. I executed ./gradlew check as far as I know it executes the tests.