chore: bump version.py to 2.3.0.dev and refresh RELEASE_MEMO

- src/datajoint/version.py: 2.2.4 → 2.3.0.dev. The GitHub Actions
  release flow auto-overwrites this at tag time; the dev marker
  reflects the current development target between releases.

- RELEASE_MEMO.md: refresh the branch table from "2.1.x" (stale; predated
  the 2.1 and 2.2 release lines) to "2.3.x".

Part of T18 (cross-cutting release plumbing).

Author: dimitri-yatsenko

RELEASE_MEMO.md

@@ -4,7 +4,7 @@
 
 | Branch | Purpose | Version |
 |--------|---------|---------|
-| `master` | Main development | 2.1.x |
+| `master` | Main development | 2.3.x |
 | `maint/2.0` | Maintenance releases | 2.0.x |
 
 For 2.0.x bugfixes:

src/datajoint/version.py

@@ -1,4 +1,4 @@
 # version bump auto managed by Github Actions:
 # label_prs.yaml(prep), release.yaml(bump), post_release.yaml(edit)
 # manually set this version will be eventually overwritten by the above actions
-__version__ = "2.2.4"
+__version__ = "2.3.0.dev"