chore: Remove fallback logic related to annotation name change

[Tomasz-Smelcerz-SAP]

After the GW Secret annotation name was changed from lastModifiedAt to caAddedToBundleAt, there is some code to be removed:

• code that removes the old annotation (internal/gatewaysecret/cabundle/handler.go)
• fallback on reading old annotation value in case new one is not yet created (internal/service/watcher/certificate/certificate_service.go, internal/service/watcher/certificate/certificate_service_test.go)

We should remove this only after the new code runs without errors for enough time - to migrate the annotation and "survive" few certificate rotations. The second release after 1.14.0 looks like a good target.

See also:
#3002, #3104

[c-pius]

Not part of KLM 1.16.1. Needs to wait for next KLM release.