chore(docs) add 3.1.6 changelog

[rainest]

What this PR does / why we need it:

Changelog entry. Artifact is out, this is just the docs.

Fix #6153

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[team-k8s-bot]

The backport to release/3.1.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 working tree
git worktree add .worktrees/backport-release/3.1.x release/3.1.x
# Navigate to the new working tree
cd .worktrees/backport-release/3.1.x
# Create a new branch
git switch --create backport-6187-to-release/3.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6cf727909c2fd85e4d94556fef0eb272d113be25
# Push it to GitHub
git push --set-upstream origin backport-6187-to-release/3.1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.1.x

Then, create a pull request where the base branch is release/3.1.x and the compare/head branch is backport-6187-to-release/3.1.x.