Add a config to make presence server more RFC compliant

FLEXISIP-788

Add a config to make presence server more RFC compliant

The 'support-legacy-client' config, defaulted to true, can be set to false to be more RFC-compliant, but it may break clients with liblinphone < 5.4.

Reviewer guidelines

The following requirements must be met before merging:

flexisip	done	N.A.	Tests added
	done	N.A.	Related MR pipelines succeeded
	done	N.A.	SHA1 consistency of cherry-picked commits
linphone-sdk	done	N.A.	On a tag... (Except for master, which is allowed to use yet-to-be-released versions of the SDK)
	done	N.A.	...From the corresponding branch
docs	done	N.A.	CHANGELOG up to date
	done	N.A.	wiki (public documentation) up to date