Release 1.8.1

Fixes:
* upgrade: do apk-tools upgrade first

Other:
* CI: Add v24.12 (MR 18)
* CI: Drop v24.06 (EOL)
* CI: add monitoring to irc/matrix
* CI: bail out if ssh host key not set
* CI: create monitoring.txt file with "OK" if no errors
* upgrade: add err_restore_apk_repos_exit func
* CI: generate_page: use &nbsp in HTML code
* CI: generate_page: fix failure vs. failures