Code review comment for ~danilogondolfo/netplan/+git/ubuntu:jammy_lp1997467

Thanks, LGTM, mostly.

Just a few nitpicks which we might want to fix:

d/changelog: Please mention the additional memory leak patch, otherwise the SRU team might complain about unlisted changes. E.g.:
```
  * d/p/lp1997467: set only specific origin-hint if given (LP: #1997467)
    Cherry-picked from upstream: https://github.com/canonical/netplan/pull/299
    - d/libnetplan0.symbols: Add netplan_parser_load_nullable_overrides() API
    - d/p/0008-src-parse-plug-memory-leaks-in-nullable-handling.patch backport
      upstream commit 40c53bb (memory leak fixup of PR#299)
```
=> please squash this change into your "Update changelog" commit

* cleanup the git history a bit:
=> please squash "458290c Adjust the symbol version to this release" into "aff434f d/libnetplan0.symbols: Add netplan_parser_load_nullable_overrides() API"

=> in "47c66f3 Add the last patch related to the bug-fix" please mention where this patch comes from, by using DEP-3 patch headers (https://dep-team.pages.debian.net/deps/dep3/) and potentially also mentioning it in the git commit message (and/or d/changelog).