chore(deps): bump go-pkgz/rest to v1.22.0, drop local CORS Vary workaround

v1.22.0 includes the preflight Vary fix (https://github.com/go-pkgz/rest/pull/44):
rest.CORS now adds Vary: Access-Control-Request-Method and
Access-Control-Request-Headers on preflight itself, making the local wrapper
that added them redundant. corsMiddleware now returns rest.CORS directly;
TestCorsMiddleware still asserts those preflight Vary headers, now supplied
upstream.

Also tidies the _example/memory_store module for the new version.

backend/_example/memory_store/go.mod

@@ -18,7 +18,7 @@ require (
 	github.com/aymerick/douceur v0.2.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/dlclark/regexp2/v2 v2.2.2 // indirect
-	github.com/go-pkgz/rest v1.21.0 // indirect
+	github.com/go-pkgz/rest v1.22.0 // indirect
 	github.com/go-pkgz/routegroup v1.6.0 // indirect
 	github.com/gorilla/css v1.0.1 // indirect
 	github.com/hashicorp/errwrap v1.1.0 // indirect

backend/_example/memory_store/go.sum

@@ -21,8 +21,8 @@ github.com/go-pkgz/jrpc v0.4.0 h1:oD7xiGrzDkndkuCjeHGugQXxbggLSV7O1QmHhoc5pYY=
 github.com/go-pkgz/jrpc v0.4.0/go.mod h1:JFoY3bRjRyx4M3CbEVDFQStMB1m2gmQ7OjqFK7q3kOo=
 github.com/go-pkgz/lgr v0.12.3 h1:QDug7kRkEsuQtruT9fNF5PVT2kZUqCDPc4GmsgS3fP8=
 github.com/go-pkgz/lgr v0.12.3/go.mod h1:lpCDgVvCIxBHZp8+sGCj9MPctIzKZyZ3QdE19ddqd54=
-github.com/go-pkgz/rest v1.21.0 h1:Y/C4d/TpclJJDxqnH1RAcS6Hmox0RIReAlkwMcUWXK4=
-github.com/go-pkgz/rest v1.21.0/go.mod h1:+AHzjHazq7Z3Tk/kRWOhbbAz/YZlUV40feC1Hf4NtbE=
+github.com/go-pkgz/rest v1.22.0 h1:d3XFKlmAGBiU9MQER9/n46iXpyUr8IQUtfjU8JlqkkY=
+github.com/go-pkgz/rest v1.22.0/go.mod h1:+AHzjHazq7Z3Tk/kRWOhbbAz/YZlUV40feC1Hf4NtbE=
 github.com/go-pkgz/routegroup v1.6.0 h1:44XHZgF6JIIldRlv+zjg6SygULASmjifnfIQjwCT0e4=
 github.com/go-pkgz/routegroup v1.6.0/go.mod h1:Pmu04fhgWhRtBMIJ8HXppnnzOPjnL/IEPBIdO2zmeqg=
 github.com/gorilla/css v1.0.1 h1:ntNaBIghp6JmvWnxbZKANoLyuXTPZ4cAMlo6RyhlbO8=