update ref to flexisip to 2.6.x

FLEXISIP-1090

Reviewer guidelines

The following requirements must be met before merging:

flexisip-conference	done	N.A.	Tests added
	done	N.A.	Related MR pipelines succeeded
	done	N.A.	SHA1 consistency of cherry-picked commits
flexisip-core	done	N.A.	On a tag... (Except for master)
	done	N.A.	...From the corresponding branch
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
	done	N.A.	internal specifications up to date