stackdriver: update changelog before release

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 53.3%. Comparing base (56a6f45) to head (1f9343d). Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #76 +/- ## ======================================= + Coverage 52.3% 53.3% +1.0% ======================================= Files 38 38 Lines 4967 5026 +59 ======================================= + Hits 2598 2680 +82 + Misses 2369 2346 -23 ```

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[djc]

I published 6245c6350fab3bb5815c60f8479688980c885277 as opentelemetry-stackdriver 0.20.0. If someone wants to add a tag, that would be great.

[lalitb]

I published 6245c63 as opentelemetry-stackdriver 0.20.0. If someone wants to add a tag, that would be great.

Thanks @djc. Have created the tag on this commit-id.