go.mod, README.md: use Go 1.22

Updates #11058

Change-Id: I95eecdc7afe2b5f8189016fdb8a773f78e9f5c42
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

README.md

@@ -37,7 +37,7 @@ not open source.
 
 ## Building
 
-We always require the latest Go release, currently Go 1.21. (While we build
+We always require the latest Go release, currently Go 1.22. (While we build
 releases with our [Go fork](https://github.com/tailscale/go/), its use is not
 required.)
 

go.mod

@@ -1,8 +1,6 @@
 module tailscale.com
 
-go 1.21.1
-
-toolchain go1.21.5
+go 1.22
 
 require (
 	filippo.io/mkcert v1.4.4