Upgrade elixir to 1.17.3

[stuartc]

Description

This PR [adds/changes/fixes]... (A description of your work goes here.)

Closes #__

Validation steps

1. (How can a reviewer validate your work?)

Additional notes for the reviewer

1. (Is there anything else the reviewer should know or look out for?)

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our Responsible AI Policy

Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR