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>

tka/chaintest_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"crypto/ed25519"
 	"fmt"
+	"maps"
 	"strconv"
 	"strings"
 	"testing"
@@ -198,9 +199,7 @@ func (c *testChain) recordParent(t *testing.T, child, parent string) {
 // This method populates c.AUMs and c.AUMHashes.
 func (c *testChain) buildChain() {
 	pending := make(map[string]*testchainNode, len(c.Nodes))
-	for k, v := range c.Nodes {
-		pending[k] = v
-	}
+	maps.Copy(pending, c.Nodes)
 
 	// AUMs with a parent need to know their hash, so we
 	// only compute AUMs whose parents have been computed

tka/key.go

@@ -7,6 +7,7 @@ import (
 	"crypto/ed25519"
 	"errors"
 	"fmt"
+	"maps"
 
 	"tailscale.com/types/tkatype"
 )
@@ -64,9 +65,7 @@ func (k Key) Clone() Key {
 
 	if k.Meta != nil {
 		out.Meta = make(map[string]string, len(k.Meta))
-		for k, v := range k.Meta {
-			out.Meta[k] = v
-		}
+		maps.Copy(out.Meta, k.Meta)
 	}
 
 	return out

tka/scenario_test.go

@@ -5,6 +5,7 @@ package tka
 
 import (
 	"crypto/ed25519"
+	"maps"
 	"sort"
 	"testing"
 )
@@ -36,9 +37,7 @@ func (s *scenarioTest) mkNode(name string) *scenarioNode {
 	}
 
 	aums := make(map[string]AUM, len(s.initial.AUMs))
-	for k, v := range s.initial.AUMs {
-		aums[k] = v
-	}
+	maps.Copy(aums, s.initial.AUMs)
 
 	n := &scenarioNode{
 		A:       authority,

tka/sync.go

@@ -107,7 +107,7 @@ func (a *Authority) SyncOffer(storage Chonk) (SyncOffer, error) {
 		skipAmount uint64  = ancestorsSkipStart
 		curs       AUMHash = a.Head()
 	)
-	for i := uint64(0); i < maxSyncHeadIntersectionIter; i++ {
+	for i := range uint64(maxSyncHeadIntersectionIter) {
 		if i > 0 && (i%skipAmount) == 0 {
 			out.Ancestors = append(out.Ancestors, curs)
 			skipAmount = skipAmount << ancestorsSkipShift