Deal with Github Actions deprecation warnings

[DMRobertson]

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Starting 1st June 2023 workflows using save-state or set-output commands via stdout will fail with an error.

See e.g. https://github.com/matrix-org/synapse/actions/runs/3258600088 (and likely other workflows too)

Node.js 12 actions, save-state and set-output are all "deprecated and will be disabled soon".

PRs:

• Deal with some GHA deprecation warnings #14216
• Deal with Github Actions deprecation warnings setup-python-poetry#11
• Use GITHUB_OUTPUT in CI backend-meta#13 and the follow-up matrix-org/backend-meta@1a8479c
• Fixes to release-artifacts warnings #14224
• Build wheels on macos 11, not 10.15 #14249
  • deal with osx 10.15 deprecation, see The macOS 10.15 Actions runner image will begin deprecation on 5/31/22 and will be fully unsupported by 12/1/22 for GitHub and ADO actions/runner-images#5583 and https://endoflife.date/macos
• Bump peaceiris/actions-gh-pages from 3.8.0 to 3.9.0 #14276
• Bump peaceiris/actions-mdbook from 1.1.14 to 1.2.0 #14275
• Fix CI and bump actions dependencies to deal with GHA deprecations. synapse-s3-storage-provider#83
• Deal with another batch of GHA warning messages #14313
• Use maintained action to install Rust in latest deps/twisted trunk jobs #14351
• Bump JasonEtco/create-an-issue from 2.5.0 to 2.8.1 #14607
• Bump docker actions to latest stable release sytest#1347
• Deal with GHA deprecation warnings synapse-s3-storage-provider#95 (integration test workflow needs fixing though)
• Upcoming GHA deprecations AustinScola/mypy-cache-github-action#277

[DMRobertson]

See https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/ . No deadline as of yet.

[DMRobertson]

https://github.com/Swatinem/rust-cache/releases/tag/v2.0.1 helps with some of this.

[DMRobertson]

Will need a tweak to https://github.com/AustinScola/mypy-cache-github-action too --- not sure how actively that repo is maintained.