Prepare for 12.6.0~pre1 release

[scpeters]

🎈 12.6.0~pre1 Preelease

Preparation for 12.6.0~pre1 prerelease.

Comparison to 12.5.0: sdformat12_12.5.0...sdf12

Needed by gazebosim/gz-sensors#259

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 #1132 (1d8de3e) into sdf12 (2bdad1b) will not change coverage.
The diff coverage is n/a.

❗ Current head 1d8de3e differs from pull request most recent head c3440e0. Consider uploading reports for the commit c3440e0 to get more accurate results

@@           Coverage Diff           @@
##            sdf12    #1132   +/-   ##
=======================================
  Coverage   91.67%   91.67%           
=======================================
  Files          80       80           
  Lines       13039    13039           
=======================================
  Hits        11953    11953           
  Misses       1086     1086           

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