msan-20 as protected branch builder and remove clang-16-msan builder

with the msan masking of archive.archive-big is time we can update the msan as a protected branch builder.

Examining recent failure shttps://buildbot.mariadb.org/#/builders/amd64-msan-clang-20 results
up to build 1537
1533 - problem with original PR - communicated to re-record tests
1531/32 - UltraGreed - MDEV-35770 Augment DBUG_ASSERT with __builtin_assume #3944 (WIP draft)
1525/1526/1529/30 - Oleg MDEV-35856: implement index hints #4058 - based on old branch base - so still archive-big test failure
1523 - Vlad - maria.maria-purge timeout in shutdown unrelated to code. only 1 occurance despite bb-10.11-wlad-aws-sdk branch pushed multiple times - similar but not identical to https://github.com/llvm/llvm-project/issues/128621
1520 - Kristian - old branch base
1519 - Bar - old branch base
1515/1519 -bsrikanth-mariadb - old branch base
(abandoning - seems inefficient way to assess - going by branch)

cross reference check by supported branches (ignoring archive.archive-big):
10.6 - clean
10.11 - 3 failures - look identical

• https://buildbot.mariadb.org/#/builders/867/builds/1106
• https://buildbot.mariadb.org/#/builders/867/builds/452
• https://buildbot.mariadb.org/#/builders/867/builds/1076
  msan stack unwinding on shutdown failure
  main.mysqladmin and main.statistics_upgrade_not_done tests.
  11.4 - 1 failure
• https://buildbot.mariadb.org/#/builders/867/builds/1267 - same as 10.11 on main.mysqladmin
  11.8 - 0 failures
  12.0 - 0 failures
  main - 1 failure -
• https://buildbot.mariadb.org/#/builders/867/builds/295 - 2 months ago -
  client.client-env-variable - failed to execute mariadb-slap (No such file or directory) - not a msan problem - assumed to be corrected