Bump recommended Go development version in README

[sigstore#568](sigstore#568) bumps the Go version
for tests to 1.17. Now they break for 1.16:

```sh
TMPDIR=$(mktemp -d)
cd $TMPDIR
git clone https://github.com/sigstore/cosign 2> /dev/null
cd cosign

for go in go_1_16 go_1_17; do
  nix-shell \
    -p ${go} \
    -p gnumake \
    --command 'make test' \
    > /dev/null 2>&1
  if [ $? -eq 0 ]; then
    echo ${go} good
  else
    echo ${go} bad
  fi
done
```

[sigstore#1252](sigstore#1252) adds a call to
`testing.T.Setenv`, added in
[1.17](https://go-review.googlesource.com/c/go/+/326790/3/api/go1.17.txt),
which caused this breakage.

(We're still testing *builds* on 1.16, so that's fine.)

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