Consolidate linux and windows containerd config templates

[brandond]

Proposed Changes

Consolidate linux and windows containerd config templates.

There was no reason for these to ever be separated. 90% of the config in the Windows template was just copy-pasted default values. The few things that DO need to be different on Windows can be handled by existing template vars.

The windows template also had the wrong structure for registry auth, which broke authenticated pulls on windows.

Types of Changes

• tech debt
• bugfix

Verification

• Note lack of separate os-specific templates
• Will need to be tested in rke2 windows.

Testing

Linked Issues

• Consolidate linux and windows containerd config templates #11693

User-Facing Change

The containerd config templates for linux and windows have been consolidated and are no longer os-specific.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 30 lines in your changes missing coverage. Please review.

Project coverage is 47.61%. Comparing base (f78769d) to head (134b08e).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/agent/config/config.go	21.05%	10 Missing and 5 partials ⚠️
pkg/agent/config/config_linux.go	42.30%	14 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11707      +/-   ##
==========================================
- Coverage   49.89%   47.61%   -2.28%     
==========================================
  Files         185      185              
  Lines       19353    19360       +7     
==========================================
- Hits         9656     9219     -437     
- Misses       8300     8805     +505     
+ Partials     1397     1336      -61     

Flag	Coverage Δ
e2etests	40.34% <33.33%> (-3.79%)	⬇️
inttests	34.99% <24.44%> (-0.07%)	⬇️
unittests	17.04% <4.44%> (+<0.01%)	⬆️

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