fix(ack): controllers only skip isUpToDate on deletion

[wotolom]

Description of your changes

• adapts the ACK controller template to only skip isUpToDate() on resource deletion (and not also postObserve())
• for docdb.Cluster
  • adapt unit-test
  • in postObserve() add if meta.WasDeleted(cr) {...} around secretRef accessing to avoid MR deletion stuck on missing secret
• also cleaned-up the no longer needed if meta.WasDeleted(cr) {return} in some isUpToDate()

Fixes #1978

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

unit-tests, manually (secretsmanager.Secret + docdb.Cluster)

[MisterMX]

LGTM. Thank you very much @wotolom for pointing out this issue and for providing the fix!