Bump version to 21.0.0git (#124870)

Also clear the release notes.

GitOrigin-RevId: 3bd3e06f3fe418e24af65457877f40cee0544f9d
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 941b029..a0b3751 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -1,5 +1,5 @@
 =======================================
-PSTL 20.0.0 (In-Progress) Release Notes
+PSTL 21.0.0 (In-Progress) Release Notes
 =======================================
 
 .. contents::