scripts: Install arm64 rspamd from official repository

[p3dda]

scripts: Install rspamd from official repository instead of debian backports on arm64 architecture

Description

Since arm64 binary packages are available on the official respository, this may be used for arm64 architecture in favor of debian backports which contain considerably older version.

Fixes #3374

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[georglauterbach]

LGTM and thank you for the update! 3 more TODOs:

1. I think we do not need deb-src, so we can safely remove it.
2. Please update our docs: there is a note about the version discrepancy between AMD64 and ARM64; just remove it altogether.
3. Please update CHANGELOG.md

[p3dda]

You are right, deb-src repository is not needed in this place, I have removed it.
CHANGELOG.md and rspamd documentation have been updated as suggested

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: bdf23fe

[georglauterbach]

This will land on :edge first and will probably go into v13.1.0.