batcher,proposer: Add gitignore (#2600)

The gitignore was non-existant or ignoring the old binary.
This updates them to hide the `bin` directory (where builds
are placed).
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

op-batcher/.gitignore

+bin

op-proposer/.gitignore

-/l2os
+bin