MINOR: Update 3.8 documentation for Kafka Streams

[lucasbru]

All public interface changes should be briefly mentioned in the
upgrade guide.

Committer Checklist (excluded from commit message)

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

[lucasbru]

@ableegoldman @nicktelford @calmera We need to update the documentation based on your KIPs. Mostly I was able to copy the docs from the KIPs, so instead of pinging everyone individually, I did it myself. Feel free to leave comments.

[nicktelford]

LGTM, thanks @lucasbru

[ableegoldman]

note: we haven't removed this config yet, planning to do so after 3.8

Might still be worth mentioning it here to encourage anyone who was using it before to switch over to the new config/the new StickyTaskAssignor interface. So we can keep this wording but just change it from "We removed the internal config..." to something like "We introduced a new config to replace the internal config ..."

[ableegoldman]

update: we will be renaming the old StickyTaskAssignor class to LegacyStickyTaskAssignor (if/when this PR is merged) so perhaps we should just say something like this:

"If you were using the old internal config internal.task.assignor.class to plug in a task assignor, please migrate your code to the new public config and pass in an assignor that implements the new TaskAssignor interface"

[lucasbru]

I'll wait with the documentation updates once the renamings have settled in the 3.8 branch

[ableegoldman]

@lucasbru alright, just merged that PR to 3.8. Here's where 3.8 stands:

1. has not removed the old internal.task.assignor config, though we want to encourage anyone using that to migrate to the new public config instead. should say something like "If you were using the old internal.task.assignor.class config, you should switch to using the new task.assignor.class config instead"
2. renamed the old TaskAssignor interface (which goes with the old internal.task.assignor config) to the LegacyTaskAssignor (don't need to say anything about this in the docs, just adding it for context)
3. renamed the old StickyTaskAssignor (which implements the LegacyTaskAssignor) to the LegacyStickyTaskAssignor, so "StickyTaskAssignor" now refers to (only) the new version of this assignor which implements the new TaskAssignor interface. For the docs, we should mention something like "If you were previously plugging in the StickyTaskAssignor via the old internal.task.assignor.class config, you will need to make sure that you are importing the new version of the StickyTaskAssignor when you switch over to the new task.assignor.class config"

[lucasbru]

I updated the migration guide. Feel free to suggest edits.

[ableegoldman]

Few more minor fixes but in general this is looking good