fix(ubuntu) delete 19.10 and 20.10 from document because unsupported

[sadayuki-matsuno]

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What did you implement:

delete 19.10 and 20.10 from document because unsupported

Type of change

Please delete options that are not relevant.

• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES

Reference

• https://blog.github.com/2015-01-21-how-to-write-the-perfect-pull-request/