feat: parameterise hardcoded rhocp-4.21 repo name across all EE versions

Problem

All three EE versions (2.4, 2.5, 2.6) hardcode --enablerepo=rhocp-4.21-for-rhel-9-*-rpms in both prepend_builder and prepend_final PKGMGR_OPTS:

- ENV PKGMGR_OPTS "--nodocs --setopt install_weak_deps=0 --enablerepo=rhocp-4.21-for-rhel-9-*-rpms"

When the OCP version needs to change, this requires manual edits in 6 places across 3 files simultaneously, creating drift risk and easy-to-miss mistakes.

Proposed Solution

Introduce a build-arg OCP_VERSION (defaulting to 4.21) and replace the hardcoded string:

prepend_builder:
  - ARG OCP_VERSION=4.21
  - ENV PKGMGR_OPTS "--nodocs --setopt install_weak_deps=0 --enablerepo=rhocp-${OCP_VERSION}-for-rhel-9-*-rpms"

Pass the arg from the CI workflow matrix to make the OCP dependency explicit and easily updated in one place.

Files to Change

• openshift-virtualization-migration-ee-2.4/execution-environment.yml
• openshift-virtualization-migration-ee-2.5/execution-environment.yml
• openshift-virtualization-migration-ee-2.6/execution-environment.yml
• .github/workflows/build-ee.yml (pass OCP_VERSION as a build-arg)

Read the three execution-environment.yml files and .github/workflows/build-ee.yml, then trace how the workflow matrix invokes the image builds. Done means OCP_VERSION is defined with the stated default in each EE definition, supplied by CI, and the six hardcoded repository references are replaced.