[LCB] Support nested changelog structures

[gunnar-solo]

Per discussion doc, changelog bot should support nested versions. This looks like:

v1.9
  v1.9.0
  v1.9.1
  v1.9.2
v1.10
  v1.10.1
  v1.10.2
  v1.10.3
  ...

Notably, this looks like almost the exact same solution as currently exists, only with the ability to specify a subfolder for processing. Known TODOs:

• [x] figure out how “release notes” are generated, and ensure solution does not break them
• [x] update changelog bot to be able to specify a subdirecty to parse, rather than a hardcoded changelog/**
  • [x] ensure said update is opt-in vai feature flag
• [x] validate a test release in a private repository
• [ ] port gloo and solo-projects to new system
• [ ] make some kind of “hey, you can do this too” post to an @here equivalent

[gunnar-solo]

this was blocked for ~2 months due to missing permissions in the build bot repo--moving back to triage, since it seems to be low priority

[nfuden]

Also new impl with github actions will include this in dod