Write a HOWTO migrate Codec format version

[slow-J]

Description

The change to changing PFOR encoding to FOR for doc blocks in #12741, required bumping the Codec version from 95 to 99.

Bumping the Codec is not a straightforward process since it involved:

• Forking all modified changed files and bumping the version number
• Moving the original copies of the modified files to backwards_codecs
• Re-generating ForUtil
  • + changes to forUtil.gradle to bump version and add a new task for regenerating backwards_codec
• Forking PostingsFormat subclasses
• Changes to tests:
  • Bumping existing Codec unit tests
  • Creating RW versions of old Codec and Codec components
  • Bumping Codec components where used throughout other tests

We required a few follow-ups to clean up the initial misses:

• Re-adding the backward_codecs.lucene90 TestPForUtil + TestForUtil #12781
• javadocs cleanup in Lucene99PostingsFormat #12776
• Add tests for the 9.0->9.8 block tree terms dict format back. #12908
• Add tests for Lucene90PostingsFormat back #12904

@mikemccand mentioned in #12904 (comment)

Codec format migrations are so tricky ... I wish we had a HOWTO somewhere.

Since I done the last Codec bump, I can have a look at writing up the documentation so subsequent codec format bumps will go smoother.

[shubhamvishu]

Nice....This would be really helpful!