Bump version to v0.23.1

Update CHANGELOG with the release number

Merge pull request libgit2#3290 from libgit2/cmn/filter-doc

filter: add docs for streaming filters

CHANGELOG: add submodule changes

Merge pull request libgit2#2815 from ethomson/example

clone example: don't divide by zero

Reformat the changelog

This should provide a easier way to see what kinds of changes we have,
and a single place to look at the breaking changes.

Fix assert when receiving uncommon sideband packet

Merge pull request libgit2#2801 from ethomson/changelog

CHANGELOG: add missing 0.22 changes

index tests: test capitalization before mkdir

Bump version to 0.21.2