Prepare for 9.9.0~pre1 prerelease

[scpeters]

🎈 Release

Preparation for 9.9.0~pre1 prerelease.

Comparison to 9.9.0~pre1: sdformat9_9.8.0...sdf9

Needed by gazebosim/gz-sensors#249

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 #1134 (6cbbd81) into sdf9 (089aa03) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf9    #1134   +/-   ##
=======================================
  Coverage   87.81%   87.81%           
=======================================
  Files          64       64           
  Lines       10099    10099           
=======================================
  Hits         8868     8868           
  Misses       1231     1231           

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