plot-bamstats: `set style increment user`

Hello!

Thank you for developing samtools!

 #### Please describe your environment.
- OS: Ubuntu 24.04 LTS
- Linux: 6.11.0-26-generic x86_64
- gnuplot: 6.0 patchlevel 0
- samtools: 1.13

```
$ plot-bamstats -p "${plots_path}" "${stats_file}"

set style increment user
                    ^
"./indel-cycles.gp" line 9: warning: deprecated command
```