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>

tsd/tsd.go

@@ -226,8 +226,7 @@ func (p *SubSystem[T]) Set(v T) {
 			return
 		}
 
-		var z *T
-		panic(fmt.Sprintf("%v is already set", reflect.TypeOf(z).Elem().String()))
+		panic(fmt.Sprintf("%v is already set", reflect.TypeFor[T]().String()))
 	}
 	p.v = v
 	p.set = true
@@ -236,8 +235,7 @@ func (p *SubSystem[T]) Set(v T) {
 // Get returns the value of p, panicking if it hasn't been set.
 func (p *SubSystem[T]) Get() T {
 	if !p.set {
-		var z *T
-		panic(fmt.Sprintf("%v is not set", reflect.TypeOf(z).Elem().String()))
+		panic(fmt.Sprintf("%v is not set", reflect.TypeFor[T]().String()))
 	}
 	return p.v
 }