Prevent extra `SET time zone` in `configure_connection` (#28413)

`SET time zone 'value'` is an alias for `SET timezone TO 'value'`.

https://www.postgresql.org/docs/current/static/sql-set.html

So if `variables["timezone"]` is specified, it is enough to
`SET timezone` once.

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -688,18 +688,20 @@ def configure_connection
           # Use standard-conforming strings so we don't have to do the E'...' dance.
           set_standard_conforming_strings
 
+          variables = @config.fetch(:variables, {}).stringify_keys
+
           # If using Active Record's time zone support configure the connection to return
           # TIMESTAMP WITH ZONE types in UTC.
-          # (SET TIME ZONE does not use an equals sign like other SET variables)
-          if ActiveRecord::Base.default_timezone == :utc
-            execute("SET time zone 'UTC'", "SCHEMA")
-          elsif @local_tz
-            execute("SET time zone '#{@local_tz}'", "SCHEMA")
+          unless variables["timezone"]
+            if ActiveRecord::Base.default_timezone == :utc
+              variables["timezone"] = "UTC"
+            elsif @local_tz
+              variables["timezone"] = @local_tz
+            end
           end
 
           # SET statements from :variables config hash
           # https://www.postgresql.org/docs/current/static/sql-set.html
-          variables = @config[:variables] || {}
           variables.map do |k, v|
             if v == ":default" || v == :default
               # Sets the value to the global or compile default

activerecord/test/cases/adapters/postgresql/connection_test.rb

@@ -220,6 +220,13 @@ def test_set_session_variable_default
       end
     end
 
+    def test_set_session_timezone
+      run_without_connection do |orig_connection|
+        ActiveRecord::Base.establish_connection(orig_connection.deep_merge(variables: { timezone: "America/New_York" }))
+        assert_equal "America/New_York", ActiveRecord::Base.connection.query_value("SHOW TIME ZONE")
+      end
+    end
+
     def test_get_and_release_advisory_lock
       lock_id = 5295901941911233559
       list_advisory_locks = <<-SQL