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/taildrop/fileops_fs.go

@@ -101,7 +101,7 @@ func (f fsFileOps) Rename(oldPath, newName string) (newPath string, err error) {
 	wantSize := st.Size()
 
 	const maxRetries = 10
-	for i := 0; i < maxRetries; i++ {
+	for range maxRetries {
 		renameMu.Lock()
 		fi, statErr := os.Stat(dst)
 		// Atomically rename the partial file as the destination file if it doesn't exist.