Sync sublogger level with parent

[mpalmi]

With the following commits, subloggers are now able to set their own levels while maintaining a hierarchical relationship to their parent/children.

https://github.com/hashicorp/go-hclog/pull/134 https://github.com/hashicorp/go-hclog/pull/137

This allows us to keep leaf sublogger levels in sync with their parent, unless specifically set otherwise.

As a result, we no longer have to track every leaf logger in the (Core).allLoggers slice. Instead, we can track main subsystem subloggers in allLoggers (e.g. core.replication), without having to manage every child (e.g. core.replication.index).

[github-actions[bot]]

CI Results: Failures:

Test Type	Package	Test	Logs
	vault	TestSystemBackend_LoggersByName	view test results
	vault	TestSystemBackend_LoggersByName/loggers-by-name-core	view test results
	vault	TestSystemBackend_LoggersByName/loggers-by-name-identity	view test results