Update milestone section in contributing.md (#7213)

Current info is not up to date as we are now using a train release model and frequently end up with PRs and issues in the milestone that are not resolved before release. As the release milestone is now mostly used for prioritizing what work gets done I updated this section to be about prioritizing PRs/issues instead of preparing releases.

CONTRIBUTING.md

@@ -137,9 +137,14 @@ Some useful pull request queries:
 [`S-Controversial`]: https://github.com/bevyengine/bevy/pulls?q=is%3Aopen+is%3Apr+label%3AS-Controversial
 [`D-Complex`]: https://github.com/bevyengine/bevy/pulls?q=is%3Aopen+is%3Apr+label%3AD-Complex
 
-### Preparing Releases
+### Prioritizing PRs and issues
 
-We track issues and pull requests that must be included in releases using [Milestones](https://github.com/bevyengine/bevy/milestones).
+We use [Milestones](https://github.com/bevyengine/bevy/milestones) to track issues and PRs that:
+
+* Need to be merged/fixed before the next release. This is generally for extremely bad bugs i.e. UB or important functionality being broken.
+* Would have higher user impact and are almost ready to be merged/fixed.
+
+There are also two priority labels: [`P-Critical`](https://github.com/bevyengine/bevy/issues?q=is%3Aopen+is%3Aissue+label%3AP-Critical) and [`P-High`](https://github.com/bevyengine/bevy/issues?q=is%3Aopen+is%3Aissue+label%3AP-High) that can be used to find issues and PRs that need to be resolved urgently.
 
 ## Making changes to Bevy