chore: remove env from mimir rules

chore: remove env from mimir rules

Resolves gitlab-com/gl-infra/scalability#3605 (closed)

• Removes env from mimir-rules/ autogenerated rules.
• Moved the handcrafted rules in mimir-rules/{tenant}/{env}/... to mimir-rules/{tenant}/...
• Moved the hardcoded path in stage-group-monthly-availability.jsonnet to follow suit. This will help gitlab-com/gl-infra/scalability#3611 (closed) where we'll define the list of tenants to generate the monthly availability recording rules.