[chart] mysql: Set version to "5.7.34-debian-10-r55" because "5.7" is broken atm

Author: geropl

chart/values.yaml

@@ -491,7 +491,8 @@ mysql:
   enabled: true
   fullnameOverride: mysql
   image:
-    tag: 5.7
+    # We switched to the specific version because "5.7" was broken at least once
+    tag: 5.7.34-debian-10-r55
   primary:
     extraEnvVars:
       # We rely on this in our DB implementations: NULL (re-)sets configured columns to be initialized with CURRENT_TIMESTAMP.