[improvement] set an explicit struct for dataDisks instead of a map

[eljohnson92]

What this PR does / why we need it:
This PR enables the nomaps linter which requires us to change the spec.dataDisks from a map[string]instanceDisks to an explicit list of instance disks. despite this stricter API spec change it should not be a breaking change since our webhooks were validating that the only dataDisks specified were the ones updated in this PR anyways.
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

❌ Patch coverage is 95.39474% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.21%. Comparing base (eaa5b92) to head (85a0418).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...nal/controller/linodemachine_controller_helpers.go	94.16%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
+ Coverage   64.76%   65.21%   +0.44%     
==========================================
  Files          71       71              
  Lines        7871     7931      +60     
==========================================
+ Hits         5098     5172      +74     
+ Misses       2465     2456       -9     
+ Partials      308      303       -5     

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