Deprecate modified indexes in TableDiff

[morozov]

The same reasoning applies as in #6827.

The @throws Exception annotations being removed from AbstractMySQLPlatform seem to have been mistakenly introduced as a result of conflict resolution during a merge up (2.11.x → 3.0.x).

See the details about the changes in PostgreSQLPlatform inline.

[morozov]

This deprecation is not as smooth as the previous one. The change in this condition is needed to prevent SchemaManagerFunctionalTestCase#testSwitchPrimaryKeyOrder() from failing. Technically, this is a bug.

The following is happening under the hood:

1. As part of this deprecation, instead of adding the new index to the "modified" list, the comparator adds the old and the new indexes to the corresponding collections.
2. In the test, the new index is created as part of the new table definition, so its name is "primary"; the old index is introspected from the database, and its name is "<table>_pkey".
3. When the schema manager attempts to drop the old index, the hack that compares the index name with "primary" no longer kicks in (previously, the schema manager would drop the new index, and now it drops the old one).

The whole point of this deprecation is to lay the groundwork for the introduction of the PrimaryKeyConstraint as the first-class citizen, which will allow to get rid of this hack and implement proper schema management on all platforms. So I believe, the risk of breaking some edge case (and then fixing it in a subsequent patch release) is justified.

An alternative might be to have the comparator add the new index both to the old and new index collections (to mimic the old behavior), but that would end up being a Postgres-specific hack in the platform-agnostic comparator, which would have to be fixed (BC-broken) later.