chore: Update notification engine to 2fef5c9

[ayatk]

Update argoproj/notifications-engine@2fef5c9

changes:

• feat(slack): Support set username and icon from template in slack notifications-engine#340

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (master@20e2e78). Learn more about missing BASE report.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20276   +/-   ##
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  Coverage          ?   55.95%           
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  Files             ?      322           
  Lines             ?    44673           
  Branches          ?        0           
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  Hits              ?    24996           
  Misses            ?    17087           
  Partials          ?     2590           

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[nitishfy]

LGTM!