ci: using ubuntu-20.04 by default

[tao12345666333]

Signed-off-by: Jintao Zhang zhangjintao9020@gmail.com

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

GitHub Actions is modifying its default system version, which may cause CI to fail since cgroup v2 will be used in the latest system, but not all Kubernetes versions support it.

Setting to ubuntu-20.04 will use cgroup v1, which is more general.

ref: https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #1504 (943571a) into master (768a35f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1504   +/-   ##
=======================================
  Coverage   41.47%   41.47%           
=======================================
  Files          85       85           
  Lines        7394     7394           
=======================================
  Hits         3067     3067           
  Misses       3974     3974           
  Partials      353      353           

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