Chore: Update deprecated default M4 type to M5

[nikolasrieble]

Background

On the 29th of March, AWS has disabled new creation of M4 type instances of RDS. Instead, the recommendation to use M5 type instances.
https://repost.aws/articles/AR_Rw2Xbe5Q9O1ZPKbU5S4Mg/amazon-rds-for-sql-server-is-ending-support-for-m4-r4-and-t2-database-instances-on-march-29-2024

In this PR we update the db size default value.

How has this been tested

I did not test it directly, but instead, verified that there are no orderable options for "db.m4.xlarge"

aws rds describe-orderable-db-instance-options \
    --region "us-west-2" \
    --db-instance-class "db.m4.xlarge" \
    --engine postgres

and that on the other hand, there are plenty options with "db.m5.xlarge", including the current default version 12.15.

aws rds describe-orderable-db-instance-options \
    --region "us-west-2" \
    --db-instance-class "db.m5.xlarge" \
    --engine postgres