fix: update ubuntu to 20.04 in pipeline

[serena-ruan]

Related Issues/PRs

None

What changes are proposed in this pull request?

Update ubuntu to 20.04 in pipeline as 18.04 is deprecated

How is this patch tested?

• I have written tests (not required for typo or doc fix) and confirmed the proposed feature/bug-fix/change works.

Does this PR change any dependencies?

• No. You can skip this section.
• Yes. Make sure the dependencies are resolved correctly, and list changes here.

Does this PR add a new feature? If so, have you added samples on website?

• No. You can skip this section.
• Yes. Make sure you have added samples following below steps.

1. Find the corresponding markdown file for your new feature in website/docs/documentation folder.
   Make sure you choose the correct class estimators/transformers and namespace.
2. Follow the pattern in markdown file and add another section for your new API, including pyspark, scala (and .NET potentially) samples.
3. Make sure the DocTable points to correct API link.
4. Navigate to website folder, and run yarn run start to make sure the website renders correctly.
5. Don't forget to add <!--pytest-codeblocks:cont--> before each python code blocks to enable auto-tests for python samples.
6. Make sure the WebsiteSamplesTests job pass in the pipeline.

AB#1937581

[serena-ruan]

/azp run

[github-actions]

Hey @serena-ruan 👋!
Thank you so much for contributing to our repository 🙌.
Someone from SynapseML Team will be reviewing this pull request soon.
We appreciate your patience and contributions 💯!

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[serena-ruan]

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[codecov-commenter]

Codecov Report

Merging #1624 (59b4d91) into master (dbbe681) will decrease coverage by 1.14%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1624      +/-   ##
==========================================
- Coverage   83.47%   82.33%   -1.15%     
==========================================
  Files         286      276      -10     
  Lines       15064    14275     -789     
  Branches      745      745              
==========================================
- Hits        12575    11753     -822     
- Misses       2489     2522      +33     

Impacted Files	Coverage Δ
...rosoft/azure/synapse/ml/param/DataFrameParam.scala	53.57% <0.00%> (-21.43%)	⬇️
.../microsoft/azure/synapse/ml/param/ModelParam.scala	14.28% <0.00%> (-85.72%)	⬇️
...t/azure/synapse/ml/param/EstimatorArrayParam.scala	7.14% <0.00%> (-85.72%)	⬇️
...azure/synapse/ml/param/TransformerArrayParam.scala	7.69% <0.00%> (-84.62%)	⬇️
...ft/azure/synapse/ml/param/PipelineStageParam.scala	33.33% <0.00%> (-66.67%)	⬇️
...osoft/azure/synapse/ml/param/TypedArrayParam.scala	0.00% <0.00%> (-41.67%)	⬇️
...ft/azure/synapse/ml/param/ArrayParamMapParam.scala	5.26% <0.00%> (-36.85%)	⬇️
...rosoft/azure/synapse/ml/param/EstimatorParam.scala	27.27% <0.00%> (-27.28%)	⬇️
...oft/azure/synapse/ml/param/UntypedArrayParam.scala	37.50% <0.00%> (-21.88%)	⬇️
...rosoft/azure/synapse/ml/param/EvaluatorParam.scala	8.33% <0.00%> (-16.67%)	⬇️
... and 21 more

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[serena-ruan]

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[serena-ruan]

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[serena-ruan]

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[serena-ruan]

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[serena-ruan]

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Azure Pipelines successfully started running 1 pipeline(s).

[serena-ruan]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).