[core/release] Implement automated releases

[Grub4K]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

This PR implements automated pre-releases, update channels and simplifies releases.

Automatic builds will be generated at each push, and listed as pre-release.
Only the last such release will be kept under /releases - all pre-releases will be archived in a separate repo.
The update mechanism is changed so that releases/pre-releases will only update to their respective channel.
You can override the update target using --update-to.

Changes

• Add devscripts/make_changelog.py for automated changelog generation
• Add release.yml, the equivalent to the old build.yml
  • The dependency on yt-dlp/ssh-agent has been removed
• Add release-nightly.yml, running and releasing for each commit
  • Closes [Discussion] Automated Daily Releases #1839
• Add publish.yml, a helper workflow for publishing releases
  • The dependency on yt-dlp/action-gh-release has been removed
• Rework build.yml to allow generation of artifacts without triggering a full release
• Add signing for the checksum files, using detached signature files
  • A public.key file for verification is commited to version control
  • Closes Provide signatures for release SHA2-256SUMS and SHA2-512SUMS #6344
• Some updater additions and fixes
  • Added --update-to flag
  • Added GitHub API headers
  • Added better error messages
    • Closes Can't update from -U #6293
• Updated documentation to reflect those changes

Before merging, the following should be set:

vars.PUSH_VERSION_COMMIT: Push a version update commit to master during release workflow
vars.BUILD_NIGHTLY: Create a nightly release on every push to master branch
secrets.GPG_SIGNING_KEY: Private GPG key used for signing SHA files

# Both need to be set to push build to archive repo as release:
vars.ARCHIVE_REPO: Repository for archiving pre-releases, e.g. `yt-dlp/yt-dlp-nightly-builds`
secrets.ARCHIVE_REPO_TOKEN: Personal Access Token with contents:write permission for archive repo

# Publish to PyPI and BREW
secrets.PYPI_TOKEN: Token for PyPI
secrets.BREW_TOKEN: Private deploy key for Homebrew taps repo

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)

[pukkandan]

Merged in multiple commits

[Kissaki]

Only the last such release will be kept under /releases - all pre-releases will be archived in a separate repo.

For GitHub release notifications this still leads to a daily notification though, right?
(I'd like to receive stable release notifications but not frequent pre-release notifications - but GitHub does not differentiate.)

[pukkandan]

There isn't really anything we can do about that

Edit: Other than not releasing it in main repo at all?