Bump recommended Go development version in README (sigstore#1340)

CI only runs tests on 1.17 (it still runs *builds* on 1.16, so the other
references in the README are fine). Further, builds *are* broken on
1.16 right now.

Signed-off-by: Zachary Newman <z@znewman.net>

README.md

@@ -30,7 +30,7 @@ For Linux and macOS binaries see the [GitHub release assets](https://github.com/
 
 ## Developer Installation
 
-If you have Go 1.16+, you can setup a development environment:
+If you have Go 1.17+, you can setup a development environment:
 
     $ git clone https://github.com/sigstore/cosign
     $ cd cosign