feat(docdb): allow (major) version upgrades

[wotolom]

Description of your changes

for DBCluster.docdb:

• add allowMajorVersionUpgrade parameter to API (same update logic as for DBCluster.rds)
• add check for engineVersion change (copied logic from rds)

Edit: 2nd commit:

• adapt unit-tests
• add engineVersion to observation
• avoid sending unnecessary Modify-class while DBCluster being in modifying/upgrading states

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

manually

[MisterMX]

LGTM. Thank you very much @wotolom!