ci: automatically assign missing priority labels to ruleset issues

What does this MR do?

Introduces a new CI script that fetches SASTRuleset issues on gitlab-org/gitlab with missing ~SAST::Ruleset::P* priority label. The script attempts to find references to either a community rule, or a rule in this project, to fetch the rule YAML and assign a priority label based on the rule's severity:

• ERROR -> SAST::RulesetP2
• WARNING -> SAST::RulesetP3
• INFO -> SAST::RulesetP4
• Other -> SAST::RulesetP4

The MR also introduces a CI job and a new maintenance stage which will be set up to run the script on a daily schedule.