KAFKA-18674: Document the incompatible changes in parsing --bootstrap-server

[mingyen066]

Based on the discussion of #18741 (comment), add the description to docs/upgrade.html

The change in behavior requiring strict validation seems reasonable for 4.0 so we probably ought to mention it in the documentation in docs/upgrade.html.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[chia7712]

@mingyen066 could you please copy the unit test of #18767 to 4.0? the other format should be failed in 4.0

[mingyen066]

Hi @chia7712 , absolutely! I've created PR #18781, which includes a unit test for verify invalid format in 4.0. PTAL. Thank you!"