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>

feature/conn25/conn25.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"net/http"
 	"net/netip"
+	"slices"
 	"sync"
 
 	"go4.org/netipx"
@@ -329,13 +330,7 @@ func configFromNodeView(n tailcfg.NodeView) (config, error) {
 		selfRoutedDomains: set.Set[dnsname.FQDN]{},
 	}
 	for _, app := range apps {
-		selfMatchesTags := false
-		for _, tag := range app.Connectors {
-			if selfTags.Contains(tag) {
-				selfMatchesTags = true
-				break
-			}
-		}
+		selfMatchesTags := slices.ContainsFunc(app.Connectors, selfTags.Contains)
 		for _, d := range app.Domains {
 			fqdn, err := dnsname.ToFQDN(d)
 			if err != nil {

feature/conn25/conn25_test.go

@@ -115,7 +115,7 @@ func TestHandleConnectorTransitIPRequestMultipleTIP(t *testing.T) {
 		t.Fatalf("n TransitIPs in response: %d, want 3", len(resp.TransitIPs))
 	}
 
-	for i := 0; i < 3; i++ {
+	for i := range 3 {
 		got := resp.TransitIPs[i].Code
 		if got != TransitIPResponseCode(0) {
 			t.Fatalf("i=%d TransitIP Code: %d, want 0", i, got)