Reset changelog after 2.18.0 release

[dbwiddis]

Description

Clears out entries already in release notes.

Check List

• Commits are signed per the DCO using --signoff.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.07%. Comparing base (32b0f5f) to head (6d8a198).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #950   +/-   ##
=========================================
  Coverage     78.07%   78.07%           
  Complexity      996      996           
=========================================
  Files            99       99           
  Lines          4625     4625           
  Branches        431      431           
=========================================
  Hits           3611     3611           
  Misses          834      834           
  Partials        180      180           

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/flow-framework/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/flow-framework/backport-2.x
# Create a new branch
git switch --create backport/backport-950-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9b9ebb2bc47e4f1a621b7b6c60db51a4851b9416
# Push it to GitHub
git push --set-upstream origin backport/backport-950-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/flow-framework/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-950-to-2.x.