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>

net/netcheck/netcheck_test.go

@@ -42,8 +42,7 @@ func TestBasic(t *testing.T) {
 
 	c := newTestClient(t)
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	if err := c.Standalone(ctx, "127.0.0.1:0"); err != nil {
 		t.Fatal(err)
@@ -124,8 +123,7 @@ func TestWorksWhenUDPBlocked(t *testing.T) {
 
 	c := newTestClient(t)
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	r, err := c.GetReport(ctx, dm, nil)
 	if err != nil {
@@ -1038,8 +1036,7 @@ func TestNoUDPNilGetReportOpts(t *testing.T) {
 	}
 
 	c := newTestClient(t)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	r, err := c.GetReport(ctx, dm, nil)
 	if err != nil {