update session storage version from 5 to 6 due to fosite upgrade

A small part of the session storage changed type in the latest version
of fosite compared to the old version of fosite that we were using.
Just to be safe, update our session storage version to invalidate
any pre-existing sessions upon upgrade of Pinniped.

internal/fositestorage/authorizationcode/authorizationcode.go

@@ -33,7 +33,8 @@ const (
 	// Version 3 is when we added the Username field to the psession.CustomSessionData.
 	// Version 4 is when fosite added json tags to their openid.DefaultSession struct.
 	// Version 5 is when we added the UpstreamUsername and UpstreamGroups fields to psession.CustomSessionData.
-	authorizeCodeStorageVersion = "5"
+	// Version 6 is when we upgraded fosite in Dec 2023.
+	authorizeCodeStorageVersion = "6"
 )
 
 var _ oauth2.AuthorizeCodeStorage = &authorizeCodeStorage{}
@@ -380,5 +381,5 @@ const ExpectedAuthorizeCodeSessionJSONFromFuzzing = `{
 			"d鞕ȸ腿tʏƲ%}ſ¯Ɣ 籌Tǘ乚Ȥ2"
 		]
 	},
-	"version": "5"
+	"version": "6"
 }`