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>

hostinfo/hostinfo_linux.go

@@ -68,7 +68,7 @@ func deviceModelLinux() string {
 }
 
 func getQnapQtsVersion(versionInfo string) string {
-	for _, field := range strings.Fields(versionInfo) {
+	for field := range strings.FieldsSeq(versionInfo) {
 		if suffix, ok := strings.CutPrefix(field, "QTSFW_"); ok {
 			return suffix
 		}
@@ -110,11 +110,11 @@ func linuxVersionMeta() (meta versionMeta) {
 		if err != nil {
 			break
 		}
-		eq := bytes.IndexByte(line, '=')
-		if eq == -1 {
+		before, after, ok := bytes.Cut(line, []byte{'='})
+		if !ok {
 			continue
 		}
-		k, v := string(line[:eq]), strings.Trim(string(line[eq+1:]), `"'`)
+		k, v := string(before), strings.Trim(string(after), `"'`)
 		m[k] = v
 	}