Update Changelog for 12.0.0

[caguero]

🎈 Release

Preparation for <12.0.0> release.

Comparison to <11.2.0>: ignition-transport11_11.2.0...gz-transport12

Checklist

• Asked team if this is a good time for a release
• There are no changes to be ported from the previous major version
• No PRs targeted at this major version are close to getting in
• Bumped minor for new features, patch for bug fixes
• Updated changelog
• Updated migration guide (as needed)
• Link to PR updating dependency versions in appropriate repository in gazebo-release (as needed):

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 #361 (388f19e) into gz-transport12 (042102f) will not change coverage.
The diff coverage is n/a.

❗ Current head 388f19e differs from pull request most recent head 4f5c853. Consider uploading reports for the commit 4f5c853 to get more accurate results

@@               Coverage Diff               @@
##           gz-transport12     #361   +/-   ##
===============================================
  Coverage           88.11%   88.11%           
===============================================
  Files                  52       52           
  Lines                4848     4848           
===============================================
  Hits                 4272     4272           
  Misses                576      576           

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