Document changes and release 21.2.6-1pureos2 into byzantium

debian/changelog

+mesa (21.2.6-1pureos2) byzantium; urgency=medium
+
+  * Upload to byzantium (no change rebuild)
+  * Remaining PureOS changes:
+    - Revert "Build with llvm-12."
+    - Update libdrm-dev dependency to match PureOS version
+    - Add python3 dependency to mesa-vulkan-drivers
+
+ -- Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>  Mon, 07 Mar 2022 13:58:37 +0100
+
 mesa (21.2.6-1pureos1.1~oct1) octarine; urgency=medium
 
   * Upload to octarine