version downgrade check defeated when version differ because of v prefix

this error was seen in https://gitlab.com/sylva-projects/sylva-core/-/jobs/13072018523:

❌ ERROR: Downgrade in versions detected for upgrade scenario in vault-operator:: 1.23.4 (in main branch) is older than v1.23.0 (fix-cnpg-wal-issues-bp branch)

We can easily avoid it by having the CI check strip any leading v before doing the check.