Update to f29 branch and build only on f28, because in f29+, mysql 8.0 is default

mysql.yaml

@@ -12,9 +12,9 @@ data:
         module: [MIT]
     dependencies:
         - buildrequires:
-              platform: [-f28]
+              platform: [f28]
           requires:
-              platform: [-f28]
+              platform: [f28]
     references:
         community: https://docs.pagure.org/modularity/
         documentation: https://github.com/container-images/mariadb/
@@ -42,5 +42,5 @@ data:
         rpms:
             community-mysql:
                 rationale: MySQL package.
-                ref: 58939b89 # latest f28 branch
+                ref: a4a6473e # latest f29 branch
                 buildorder: 10