Prepare for 9.8.0 release

[WilliamLewww]

🎈 Release

Preparation for 9.8.0 release.

Comparison to 9.8.0: sdformat9_9.7.0...sdf9

Needed by <PR(s)>

• Added shininess to models gazebo-classic#3213
• Support <anti_aliasing> element when setting up camera render texture gazebo-classic#3201

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 ignition-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.

[chapulina]

LGTM once the changelog has been cleaned up 🚀

[codecov-commenter]

Codecov Report

Merging #1005 (c9a3680) into sdf9 (52ce8fb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf9    #1005   +/-   ##
=======================================
  Coverage   43.47%   43.47%           
=======================================
  Files           1        1           
  Lines          23       23           
=======================================
  Hits           10       10           
  Misses         13       13           

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