Deprecate source_macro

`source_macro` is no longer used in any ActiveRecord code. I've
chosen to deprecate it because it was not marked as nodoc and may
be in use outside of rails source.
rename MiniTest to Minitest
Merge pull request #16373 from eileencodes/deprecate-reflection-source_macro

Deprecate source_macro
Merge pull request #16374 from rajcybage/consistency_minitest

rename MiniTest to Minitest
[ci skip] fix markdown