fix: switch github-runner to be the requirer

[cbartz]

Applicable spec:

Overview

Switch the side of the relation with the planner charm

Counterpart PR of canonical/github-runner-operators#121

Rationale

The planner is the service provider. Even if it does not technically matter, per convention the service provider should be the provider.

Checklist

• The charm style guide was applied.
• The contributing guide was applied.
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex).
• The changelog is updated with changes that affects the users of the charm.
• The application version number is updated in github-runner-manager/pyproject.toml.

No app version change - no need to bump

[cbartz]

Got two approvals, the test_prometheus_metrics is not passing due to infrastructure issues - unrelated to the change in this PR.. Will force merge.