enhance(yaml): allow for users to set version legacy for buildkite

[ecrupper]

Continuation of piecemealing #1220 together with smaller PRs. This PR introduces internal.ParseYAML which will attempt to parse the pipeline or template using go-yaml v3 unless version: legacy is declared at the top level of the file.

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 56.64%. Comparing base (605647a) to head (148bdf4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/yaml.go	75.67%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1230      +/-   ##
==========================================
+ Coverage   56.63%   56.64%   +0.01%     
==========================================
  Files         622      623       +1     
  Lines       35252    35280      +28     
==========================================
+ Hits        19964    19986      +22     
- Misses      14616    14620       +4     
- Partials      672      674       +2     

Files with missing lines	Coverage Δ
compiler/native/parse.go	85.00% <100.00%> (-0.37%)	⬇️
compiler/template/native/render.go	86.56% <100.00%> (-0.39%)	⬇️
compiler/template/starlark/render.go	44.51% <100.00%> (-1.40%)	⬇️
internal/yaml.go	75.67% <75.67%> (ø)

... and 1 file with indirect coverage changes

[KellyMerrick]

lgtm