Add apk repository, update debian repository instructions

[henderkes]

As per discussion here #2060 (comment) I went ahead with different repos for different php versions. Versioned support with stuff like apt install frankenphp8.5 or apk add frankenphp85 are technically also ready, but I'm not running any CI for that yet. I don't think it's worth it at this point as it would double the amount of runs.

The old debian repository with only 8.4 is deprecated but will receive updates for a few more months.
Every update/installation will print this notice, though, which will hopefully make everyone aware:

# running update from 8.4.15...
Unpacking php-zts-cli (8.4.16-1) ...
Setting up php-zts-cli (8.4.16-1) ...

================================================================================
                          ⚠️  DEPRECATION NOTICE
================================================================================

The single-version php-zts repository is deprecated and will no longer receive updates.

Please migrate to the new repository with different PHP versions available.

More information: https://pkgs.henderkes.com

================================================================================

Updated the installer to version 8.5.

Linter failures are unrelated to this pr.

[henderkes]

I will create another pull request to install autocompletion (sh for alpine, bash for deb/rpm) with postinstall scripts after this is merged.