pkg/rootless: modernize

Using modernize (aka go fix) from Go 1.26.3, implement the following
changes:

> pkg/rootless/rootless_linux.go:319:30: fmtappendf: Replace []byte(fmt.Sprintf...) with fmt.Appendf (modernize)
> 		err = os.WriteFile(uidMap, []byte(fmt.Sprintf("%d %d 1\n", 0, os.Geteuid())), 0o666)
> 		                           ^
> pkg/rootless/rootless_linux.go:339:30: fmtappendf: Replace []byte(fmt.Sprintf...) with fmt.Appendf (modernize)
> 		err = os.WriteFile(gidMap, []byte(fmt.Sprintf("%d %d 1\n", 0, os.Getegid())), 0o666)
> 		                           ^
> pkg/rootless/rootless_linux.go:381:6: rangeint: for loop can be modernized using range over int (modernize)
> 	for sig := 0; sig < numSig; sig++ {
> 	    ^
> pkg/rootless/rootless_linux.go:432:24: stringsseq: Ranging over SplitSeq is more efficient (modernize)
> 	for _, entry := range bytes.Split(data, []byte{0}) {
> 	                      ^

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>