docs: add sha256 checksums for 10.6.4

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
(cherry picked from commit 99793e2541)

docs/relnotes/10.6.4.html

@@ -31,7 +31,8 @@ because compatibility contexts are not supported.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+4960bf17d8b5d6a6503c6954ec6cf480b5cd930797bac901c60bea192675f85e  mesa-10.6.4.tar.gz
+8f5ac103f0f503de2f7a985b0df349bd4ecdfe7f51c714be146fa5a9a3c07b77  mesa-10.6.4.tar.xz
 </pre>