RDS dbinstance: Check only major version if AutoMinorVersionUpgrade is true

[akesser]

Description of your changes

If a rds dbinstance is deployed with AutoMinorVersionUpgrade set to true, and aws updates the minor version, the controller thinks there is a change. In this pull request, the isUpToDate method is updated to check only the major version of the db in this case.

Fixes #832

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

Tested manually

[haarchri]

LGTM and tested in our environment

[andy-townsend]

I have autoMinorVersionUpgrade: true set on an RDSInstance that I've deployed as Postgres 15.2. If I manually upgrade the instance in the console to 15.3 via the AWS console (as AWS would presumably do in the next maintenance window), then the CR just goes into a Synced: False status and just see this sort of error on the CR.

Message:               update failed: cannot modify RDS instance: api error InvalidParameterCombination: Cannot upgrade postgres from 15.3 to 15.2

Is there something I've missed here as the PR seems to suggest that this shouldn't be the case now? @haarchri