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>

appc/appconnector_test.go

@@ -698,7 +698,7 @@ func TestRateLogger(t *testing.T) {
 		wasCalled = true
 	})
 
-	for i := 0; i < 3; i++ {
+	for range 3 {
 		clock.Advance(1 * time.Millisecond)
 		rl.update(0)
 		if wasCalled {
@@ -720,7 +720,7 @@ func TestRateLogger(t *testing.T) {
 		wasCalled = true
 	})
 
-	for i := 0; i < 3; i++ {
+	for range 3 {
 		clock.Advance(1 * time.Minute)
 		rl.update(0)
 		if wasCalled {