fix(ci): add compare_to to MR changes rules

MR pipelines with 'changes' directive only check the last commit by default. This causes builds to not trigger when the changed files are in earlier commits.

Add compare_to: $CI_MERGE_REQUEST_TARGET_BRANCH_NAME to all 50 MR rules with changes blocks. This compares the full MR diff against the target branch, ensuring builds trigger when OPEA/core/frontend/backend files change anywhere in the MR.