[Deprecation service] Expose level field to config deprecations

[Bamieh]

We assumed that all config deprecations are of level critical hence we did not expose this field to the deprecation factory.

A few configs are indeed not critical for 8.0 hence allowing devs to override the level make sense here. Examples:

1. Session timeout - starting in 8.0, the default timeout/lifespan will be 1h/30d (respectively). if a user does not have these values set, we advise them that they can be configured before the 8.0 release.
2. deprecated usage of elasticsearch.username - this will not break in 8.0, we will break it in the future, so I’d like to keep the deprecation message but it is not critical as it will not prevent them from upgrading
3. deprecated usage of legacy audit logs - again, this will not break in 8.0, but we will break it in the future

cc @jportner @alisonelizabeth

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)