bump envoy to 1.21.3

[wasaga]

Summary

security upgrade https://groups.google.com/g/envoy-announce/c/QxI6z6wdL7M/m/0zPrdTLAAQAJ

Related issues

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

Coverage increased (+0.04%) to 63.777% when pulling ece63b6 on wasaga/envoy-1-21-3 into a2d5d80 on main.

[backport-actions-token]

The backport to 0-17-0 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-0-17-0 0-17-0
# Navigate to the new working tree
cd .worktrees/backport-0-17-0
# Create a new branch
git switch --create backport-3413-to-0-17-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 2487e9af209352a23db1f360822539b35dd98262
# Push it to GitHub
git push --set-upstream origin backport-3413-to-0-17-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-17-0

Then, create a pull request where the base branch is 0-17-0 and the compare/head branch is backport-3413-to-0-17-0.