convert UnPack into an extension

[JonasIsensee]

No description provided.

[Copilot]

Pull Request Overview

This PR converts the UnPack compatibility into an extension and updates the lower Julia version requirement.

• Converted UnPack.jl compatibility into an extension
• Bumped lower Julia version compatibility to v1.9

Files not reviewed (2)

• ext/UnPackExt.jl: Language not supported
• src/JLD2.jl: Language not supported

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.26%. Comparing base (95c0913) to head (3162607).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
+ Coverage   84.75%   85.26%   +0.51%     
==========================================
  Files          37       37              
  Lines        4243     4399     +156     
==========================================
+ Hits         3596     3751     +155     
- Misses        647      648       +1     

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