Reject disk shrinkage on SQL instances

[nat-henderson]

Fixes #1612.

Tested with a local example, which did require ForceNew when the disk shrank but updated an increased disk size without trouble. Running cloud tests now.

[nat-henderson]

All pass, except

--- FAIL: TestAccSqlDatabaseInstance_basic3 (641.74s)
        testing.go:518: Step 0 error: Error applying: 1 error(s) occurred:

                * google_sql_database_instance.instance: 1 error(s) occurred:

                * google_sql_database_instance.instance: Error, failed to delete default 'root'@'*' user, but the database was created successfully: 

Which I think is a flake, but I'll rerun to be sure.

[nat-henderson]

Yep, it passed on the second try. :) This is ready to merge.

[ghost]

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