[deprecation] Deprecate stackscript

[Andrews2024]

What this PR does / why we need it:
This PR keeps the CloudinitMetadataSupport field in the API for backwards compatibility but otherwise removes support for stackscript. Cloud-init is sufficiently supported so that stackscript can be removed.

TODOs:

• squashed commits
• includes documentation
• updates e2e tests

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.22%. Comparing base (15be027) to head (bbf28cf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nal/controller/linodemachine_controller_helpers.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #724      +/-   ##
==========================================
- Coverage   65.38%   65.22%   -0.16%     
==========================================
  Files          70       69       -1     
  Lines        6647     6597      -50     
==========================================
- Hits         4346     4303      -43     
+ Misses       2046     2041       -5     
+ Partials      255      253       -2     

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[rahulait]

LGTM