Clear release notes for upcoming LLVM 20 dev cycle

GitOrigin-RevId: 10c6d6349e51bb245b9deec4aafca9885971135b

docs/ReleaseNotes.rst

diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 342b2ea..941b029 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -1,5 +1,5 @@
 =======================================
-PSTL 19.0.0 (In-Progress) Release Notes
+PSTL 20.0.0 (In-Progress) Release Notes
 =======================================
 
 .. contents::