Update note on MySQL index order support [ci skip]

MySQL supports descending indexes from 8.0.1 onwards:
https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-1.html

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -715,7 +715,7 @@ def rename_column(table_name, column_name, new_column_name)
       #
       #   CREATE INDEX by_branch_desc_party ON accounts(branch_id DESC, party_id ASC, surname)
       #
-      # Note: MySQL doesn't yet support index order (it accepts the syntax but ignores it).
+      # Note: MySQL only supports index order from 8.0.1 onwards (earlier versions accepted the syntax but ignored it).
       #
       # ====== Creating a partial index
       #