docs: Update release notes for 2.9.8

[JStickler]

What this PR does / why we need it:

Updates the Release Notes for version 2.9.8.

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

The backport to release-3.0.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-12881-to-release-3.0.x origin/release-3.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x a21c1077deecfb6b7a68a2ba6e472cafc2a2eca3

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-12881-to-release-3.0.x
# Create the PR body template
PR_BODY=$(gh pr view 12881 --json body --template 'Backport a21c1077deecfb6b7a68a2ba6e472cafc2a2eca3 from #12881{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "chore: [release-3.0.x] docs: Update release notes for 2.9.8" --body-file - --label "size/XS" --label "type/docs" --label "backport" --base release-3.0.x --milestone release-3.0.x --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-12881-to-release-3.0.x

# Create a pull request where the `base` branch is `release-3.0.x` and the `compare`/`head` branch is `backport-12881-to-release-3.0.x`.

# Remove the local backport branch
git switch main
git branch -D backport-12881-to-release-3.0.x