Make :register_project_runners permission only available to project owners

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Currently, only group owners can register new group runners. However, project maintainers are able to register project runners. Given Permission alignment for OWNER and MAINTAINER r... (#374607), we're aligning the permissions between group owners and project owners.

Proposal

Make :register_project_runners permission (and :create_runners on projects) only available to project owners.

Implementation steps

1. Move enable :register_project_runners from rule { can?(:maintainer_access) } block to rule { can?(:owner_access) }.policy do block.
2. Same for ProjectPolicy.
3. Update specs.