Move JUnit rendering to `ruff_db` (#19370)
Summary
--
This PR moves the JUnit output format to the new rendering
infrastructure. As I
mention in a TODO in the code, there's some code that will be shared
with the
`grouped` output format. Hopefully I'll have that PR up too by the time
this one
is reviewed.
Test Plan
--
Existing tests moved to `ruff_db`
---------
Co-authored-by: Micha Reiser <micha@reiser.io>
