build: use tretuna/sync-branches to sync alpha with master

[adamstankiewicz]

Description

Adds a Github Action workflow file for syncing alpha with master on any commit to master (using https://github.com/TreTuna/sync-branches)

Deploy Preview

N/A

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Netlify deploy preview, if applicable.
• Does your change adhere to the documented style conventions?
• Do any prop types have missing descriptions in the Props API tables in the documentation site (check deploy preview)?
• Were your changes tested using all available themes (see theme switcher in the header of the deploy preview, under the "Settings" icon)?
• Were your changes tested in the example app?
• Is there adequate test coverage for your changes?
• Consider whether this change needs to reviewed/QA'ed for accessibility (a11y). If so, please add wittjeff and adamstankiewicz as reviewers on this PR.

Post-merge Checklist

• Verify your changes were released to NPM at the expected version.
• If you'd like, share your contribution in #show-and-tell.
• 🎉 🙌 Celebrate! Thanks for your contribution.

[netlify]

✅ Deploy Preview for paragon-openedx ready!

Name	Link
🔨 Latest commit	62895e0
🔍 Latest deploy log	https://app.netlify.com/sites/paragon-openedx/deploys/63f623d51f521400089879aa
😎 Deploy Preview	https://deploy-preview-2071--paragon-openedx.netlify.app
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[codecov]

Codecov Report

Base: 90.94% // Head: 90.82% // Decreases project coverage by -0.12% ⚠️

Coverage data is based on head (62895e0) compared to base (848183e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2071      +/-   ##
==========================================
- Coverage   90.94%   90.82%   -0.12%     
==========================================
  Files         233      233              
  Lines        4174     4099      -75     
  Branches      991      971      -20     
==========================================
- Hits         3796     3723      -73     
+ Misses        371      369       -2     
  Partials        7        7              

Impacted Files	Coverage Δ
src/Button/index.jsx	73.33% <0.00%> (-10.67%)	⬇️
src/Overlay/index.jsx	81.81% <0.00%> (-3.19%)	⬇️
src/Tabs/index.jsx	92.06% <0.00%> (-1.69%)	⬇️
src/Dropdown/index.jsx	97.56% <0.00%> (-0.88%)	⬇️

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[edx-semantic-release]

🎉 This PR is included in version 21.0.0-alpha.14 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[edx-semantic-release]

🎉 This PR is included in version 20.28.5 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀