various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

tka/tka_test.go

@@ -195,7 +195,7 @@ func TestComputeStateAt(t *testing.T) {
 // provided int can be used to tweak the resulting hash (needed
 // for tests you want one AUM to be 'lower' than another, so that
 // that chain is taken based on fork resolution rules).
-func fakeAUM(t *testing.T, template interface{}, parent *AUMHash) (AUM, AUMHash) {
+func fakeAUM(t *testing.T, template any, parent *AUMHash) (AUM, AUMHash) {
 	if seed, ok := template.(int); ok {
 		a := AUM{MessageKind: AUMNoOp, KeyID: []byte{byte(seed)}}
 		if parent != nil {