rpk: generate script with sha256sum instead of md5sum

[andrewhsu]

issue https://redpandadata.atlassian.net/browse/PESDLC-1040

follow-on to PR #17568, this PR changes the way tune scripts are generated so that sha256sum is used instead of md5sum.

rpk redpanda tune disk_irq --output-script=tune_disk_irq.sh

bash$ head tune_disk_irq.sh
#!/bin/bash

# Redpanda Tuning Script
# ----------------------------------
# This file was autogenerated by RPK

file_hash=$(sha256sum /etc/default/irqbalance | cut -c -32)
cp /etc/default/irqbalance /etc/default/irqbalance.vectorized.${file_hash}.bk
cat << EOF > /etc/default/irqbalance
  # irqbalance is a daemon process that distributes interrupts across

this PR does not change dependency requirements of the user because like md5sum, sha256sum is also provided by coreutils package.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x

Release Notes

Improvements

• rpk: tune script uses sha256sum instead of md5sum