Simplify base ISO update workflow

[3alpha]

Description

Simplifies the automated Ubuntu and Debian base ISO update workflow by relying on create-pull-request for branch and PR updates. It also validates upstream ISO metadata, compares both filenames and checksums, fails on missing entries, and safely passes values to shell commands.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Other: parsed the workflow as YAML, checked every run block with bash -n, and ran git diff --check

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have checked my code and corrected any misspellings