mod+GitHub: shorten PR template, bump golang to 1.15

The golang version always needs to be in sync between the go.mod, the PR
checklist and the installation instructions.

We also shorten the Pull Request template by removing everything that we
have a CI check for and just refer to those checks instead.

go.mod

@@ -83,4 +83,6 @@ replace git.schwanenlied.me/yawning/bsaes.git => github.com/Yawning/bsaes v0.0.0
 // See https://github.com/etcd-io/etcd/issues/11154
 replace go.etcd.io/etcd => go.etcd.io/etcd v0.5.0-alpha.5.0.20201125193152-8a03d2e9614b
 
+// If you change this please also update .github/pull_request_template.md and
+// docs/INSTALL.md.
 go 1.15