Prep for release v3.9.39

[spatten]

Overview

Prep for a release to pull in some licensing changes in Themis.

https://github.com/fossas/themis/pull/81
https://github.com/fossas/themis/pull/79

I also made a minor edit to the release documentation so that I don't have to type the tag name multiple times when cutting a release.

Acceptance criteria

• The changelog is fine
• We like the new release instructions
• We pull in the new version of Themis

Testing plan

This is a doc-only change, and the licensing changes are tested in Themis, so no testing is required.

Risks

None

Metrics

Nope

References

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an ## Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.