chore: replace gardener promtail with alloy

[ma-hartma]

Description

This was split from #595, as it needs to be re-evaluated and tested separately.

TODOs

• event-exporter unneeded
• sensible defaults
• test it

Used AI-Tools ✨

• Claude Sonnet 4.6 for docs updates

Closes #552

Required Actions

The control-plane `gardener-logging` role now supports Grafana Alloy as the log collector. Alloy needs to be configured before running the new version of the role, as the deprecated `Promtail` DaemonSet is disabled and removed by default.

Follow the migration instructions:
- https://github.com/metal-stack/metal-roles/blob/master/control-plane/roles/logging-common/README.md#migration-from-promtail

Several migration scenarios are supported — from a hard cutover to a parallel run for verification before switching over. Note that running both simultaneously will produce duplicate log entries in Loki during the transition window. 

[ma-hartma]

@Gerrit91 This would be ready for a test and is also documented in metal-stack/website#260 now.