chore: Ubuntu 16.04-LTS is EOL

[jackfrancis]

Reason for Change:

This PR updates documentation to remove recommended references to the "ubuntu" and "aks-ubuntu-16.04" distro values, as we want to discourage new usages of Ubuntu 16.04-LTS, which as reached EOL.

Additionally, we will now log a warning message when a new cluster is built with a 16.04-LTS configuration.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[jackfrancis]

cc @jadarsie @haofan-ms @Michael-Sinz

Note that this PR doesn't actually deprecate Ubuntu 16.04-LTS. Its intention is to make it much less likely that someone would build a new cluster backed by 16.04-LTS (the default has been 18.04-LTS for a long time, so any new 16.04-LTS clusters would have to be explicitly configured as such).

[codecov]

Codecov Report

Merging #4418 (04012c7) into master (60828c7) will increase coverage by 0.00%.
The diff coverage is 72.66%.

@@           Coverage Diff            @@
##           master    #4418    +/-   ##
========================================
  Coverage   72.04%   72.05%            
========================================
  Files         141      141            
  Lines       21631    21764   +133     
========================================
+ Hits        15584    15681    +97     
- Misses       5096     5131    +35     
- Partials      951      952     +1     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (ø)
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/api/types.go	92.85% <ø> (ø)
pkg/api/vlabs/types.go	73.04% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
pkg/engine/template_generator.go	66.37% <11.53%> (-2.17%)	⬇️
cmd/get_logs.go	49.57% <30.43%> (-1.79%)	⬇️
pkg/api/addons.go	98.01% <100.00%> (ø)
pkg/api/converterfromapi.go	95.71% <100.00%> (+<0.01%)	⬆️
pkg/api/convertertoapi.go	94.04% <100.00%> (+0.01%)	⬆️
... and 2 more

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[Michael-Sinz]

Nice to see things removed, especially those EOL 16.04 references.

[jadarsie]

Created issue to move these out of az/akse #4423

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, jadarsie

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