all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/k8s-operator/proxygroup_test.go

@@ -1826,10 +1826,10 @@ func addNodeIDToStateSecrets(t *testing.T, fc client.WithWatch, pg *tsapi.ProxyG
 				currentProfileKey:       []byte(key),
 				key:                     bytes,
 				kubetypes.KeyDeviceIPs:  []byte(`["1.2.3.4", "::1"]`),
-				kubetypes.KeyDeviceFQDN: []byte(fmt.Sprintf("hostname-nodeid-%d.tails-scales.ts.net", i)),
+				kubetypes.KeyDeviceFQDN: fmt.Appendf(nil, "hostname-nodeid-%d.tails-scales.ts.net", i),
 				// TODO(tomhjp): We have two different mechanisms to retrieve device IDs.
 				// Consolidate on this one.
-				kubetypes.KeyDeviceID: []byte(fmt.Sprintf("nodeid-%d", i)),
+				kubetypes.KeyDeviceID: fmt.Appendf(nil, "nodeid-%d", i),
 				kubetypes.KeyPodUID:   []byte(podUID),
 			}
 		})