Revert "version up: MariaDB 10.3.28"

This reverts commit 685f611 partially
because it breaks Mroonga (mroonga/mroonga#394).
It's not so urgent to upgrade MariaDB so let's wait for a new release
of Mroonga.

component/mariadb/buildout.cfg

@@ -131,8 +131,8 @@ environment =
 ### (we just override here for easier revert)
 [mariadb-10.3]
 <= mariadb-10.4
-version = 10.3.28
-md5sum = da27b3c85abb7fe47f813ecc4d7a6a22
+version = 10.3.27
+md5sum = 6ab2934a671191d8ca8730e9a626c5c9
 post-install =
   ldd=`ldd ${:location}/lib/plugin/ha_rocksdb.so`
   for x in ${lz4:location} ${snappy:location} ${zstd:location}