[11.x] Fix dropColumnIfExists and dropForeignIfExists

[hafezdivandari]

This PR does:

• Fixes [11.x] Added dropColumnsIfExists, dropColumnIfExists and dropForeignIfExists #53305:
  • Remove direct usage of Schema Facade on Blueprint, as it ignores builder / migration's connection, checking existence on default connection and dropping on the specified connection.
  • Use native if exists statement on PostgreSQL.
  • Fix dropForeignIfExists(), could never find the constraint name and was doing nothing!
  • Add integration tests.
• Fixes [11.x] Support Optional Dimensions for vector Column Type #53316
  • Let the column definition know that dimension is explicitly null.

PS: Blueprint doesn't know about the DB connection, it just keeps commands and when Builder provides the connection and grammar instances, Blueprint converts them to SQL statements (defined on Schema\Grammar).

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[taylorotwell]

Just going to revert. All of the changes aren't really worth it imo.