Drop ROS 2 Iron support

[hidmic]

Proposed changes

ROS 2 Iron Irwini reached EOL as of late November 2024 and was officially phased out a bit over a week ago https://discourse.ros.org/t/iron-irwini-is-end-of-life/41031. As a consequence, plain rosdep resolve started failing in CI, see #452.

We have no reason to continue supporting ROS 2 Iron, thus this patch drops support for it entirely.

Type of change

• 🐛 Bugfix (change which fixes an issue)
• 🚀 Feature (change which adds functionality)
• 📚 Documentation (change which fixes or extends documentation)

Checklist

• Lint and unit tests (if any) pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• All commits have been signed for DCO

[glpuga]

LGTM, but notice there are three more mentions of iron:

• Two of them in package.xml within beluga_tools
• One in DEVELOPING.md