Fix GitLab Pages deployments not working

!20154 (merged) broke new deployments of GitLab Pages. The call to project.feature_available?(:pages_size_limit) failed with this error:

Use `License.feature_available?` for features that cannot be
restricted to only a subset of projects or namespaces

Since GLOBAL_FEATURES includes pages_size_limit, we must call License.feature_available?(:pages_size_limit) instead to be consistent with EE::GlobalPolicy.

Closes #121660 (closed)