Add PR template to highlight exoplayer v2 is deprecated

(cherry picked from commit 5b2b7f4e6147678dc56ce28826682ba21c4c9508)

.github/pull_request_template.md

+## ExoPlayer v2 is deprecated
+
+This project is deprecated. The ExoPlayer code has moved to the
+`androidx.media3` package and is published at
+https://github.com/androidx/media. Please send any PRs to that project,
+targeting the media3 `main` branch.
+
+PRs on this project will be closed with no further comment.
+