Setting log_slow_admin="" at startup should be converted to log_slow_admin=ALL

We have an issue if a user have the following in a configuration file in 10.6
log_slow_filter="" # Log everything to slow query log
log_queries_not_using_indexes=ON

This set log_slow_filter to 'not_using_index' which disables slow_query_logging of
most queries.
In effect, on should never use log_slow_filter='' in config files but instead use 'log_slow_filter=ALL'.

To fix this is in a compatible way is to change log_slow_filter="" that comes either from a configuration file or from the command line when starting to the server to log_slow_filter=ALL.

When doing the conversion, we should print a warning to inform the user that they should at some point change their configuration files to use log_slow_filter=ALL.

We should also backport from 10.6 that one can set all bits in a set variable with =ALL