Document code patch maintenance policy

[finnlewis]

We want to make it clearer how we keep track of patches for upstream code such as Drupal core and contrib. projects.

Creating this issue to track that and discuss if needed.

[finnlewis]

Following discussion in today's tech governance meeting, we now want to update the patch policy.

Specifically:

update the patch policy to target support for the latest version of Drupal core or contrib modules. This means removing a patch that has been included in the latest release. Document that the patch is needed for version 10.2 (for example) release notes and README.md

I'll update the patch policy.