LU-18319 tests: skip sanity-flr/test_50a for old MDS

LU-18319 tests: skip sanity-flr/test_50a for old MDS

Skip sanity-flr test_50a for old server because of changes that
were made on master from LU-13805 to reject unaligned DIO.

Test-Parameters: trivial
Fixes: 88d324be08 ("LU-13805 osd: Implement unaligned DIO connect flag")
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I1bbd7528de458b4cca48c466203fc5cdd0ca4e8e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56676
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Colin Faber <cfaber@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>