Migration guide: remove outdated entry

[scpeters]

🦟 Bug fix

Fixes migration guide as follow up to #454

Summary

A variant of appendToStream was deprecated in #376, but then it was removed in #454 after recalling that it hadn't yet been released. This updates the Migration guide to remove the deprecation entry for the removed method.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #499 (a2a5643) into gz-math7 (b9d69e5) will not change coverage.
The diff coverage is n/a.

❗ Current head a2a5643 differs from pull request most recent head 1865a82. Consider uploading reports for the commit 1865a82 to get more accurate results

@@            Coverage Diff            @@
##           gz-math7     #499   +/-   ##
=========================================
  Coverage     99.70%   99.70%           
=========================================
  Files            77       77           
  Lines          7007     7007           
=========================================
  Hits           6986     6986           
  Misses           21       21           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.