add changelog for 0.21.1

[ndgrigorian]

This PR adds the changelog for the 0.21.1 release

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/2204/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.21.0=py310h93fe807_12 ran successfully.
Passed: 1112
Failed: 46
Skipped: 82

[github-actions]

Array API standard conformance tests for dpctl=0.21.0=py310h93fe807_12 ran successfully.
Passed: 1111
Failed: 47
Skipped: 82