Fix generated index names for tables within schemas in PostgreSQL

[fatkodima]

Creation of indexes for tables with schemas generates incorrect index names:

connection.add_index "ci.my_table", "my_column"
(1.7ms)  CREATE INDEX "index_ci.my_table_on_my_column" ON "ci"."my_table" ("my_column")

And it is impossible then to delete this index:

connection.remove_index "ci.my_table", "my_column"
(0.4ms)  DROP INDEX  "ci"."index_ci"
PG::UndefinedObject: ERROR:  index "index_ci" does not exist (ActiveRecord::StatementInvalid)

And the same when explicitly specifying an index name:

connection.remove_index "ci.my_table", "my_column", name: "index_ci.my_table_on_my_column"
/rails/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb:466:in `remove_index': Index schema 'index_ci' does not match table schema 'ci' (ArgumentError)

Fixes #40821.