Add PR numbers to a bunch of changelog entries

[cdisselkoen]

Description of changes

I've added PR numbers to a bunch of changelog entries, mainly the ones where the PR making the change actually included the changelog entry itself. (so, the easy cases.) The remaining changelog entries will probably require more log diving if we want to include their corresponding PR numbers.

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar Dafny model or DRT infrastructure.

Disclaimer

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[john-h-kastner-aws]

sad that github doesn't seem to auto-link to the PRs.