refactor(adapter)!: drop the valkey adapter in favor of redis

Valkey speaks the Redis protocol, so the redis adapter (go-redis) already
serves it. Dropping valkey-go removes a dependency and shrinks the stripped
binary by about 9 MB.

The redis adapter now also closes a client that failed to connect only once.

BREAKING CHANGE: `adapter: valkey` is no longer accepted. Use
`adapter: redis` with a redis:// or rediss:// URL instead of valkey:// or
valkeys://. The namespace option now applies to these services.

config_test.go

@@ -33,7 +33,7 @@ func TestConfigExampleYMLParses(t *testing.T) {
 	if err != nil {
 		t.Fatalf("loadConfigFile returned error: %v", err)
 	}
-	wantAdapters := []string{"redis", "valkey", "postgresql", "mysql", "mongodb", "couchbase"}
+	wantAdapters := []string{"redis", "redis", "postgresql", "mysql", "mongodb", "couchbase"}
 	if len(services) != len(wantAdapters) {
 		t.Fatalf("len(services) = %d, want %d", len(services), len(wantAdapters))
 	}