ci: remove the Buildkite CDN release pipeline

[abueide]

Summary

Reverses #1382. CDN release is moving to a GitHub Actions-based deploy instead of Twilio Buildkite.

• Removes .buildkite/pipeline.yml
• Removes publish.yml's "Trigger CDN release build" step (Buildkite API call + BUILDKITE_API_TOKEN)
• Updates the dead Segment Buildkite CI badge in README.md to point at the GHA ci.yml workflow
• Fixes a stale (Buildkite) reference in RELEASING.md left over from before the CI migration

No replacement GHA-based S3 deploy step is added here - that's separate follow-up work.

Test plan

• Confirm publish.yml still runs end-to-end (npm publish path is unaffected by this change)

[tvs-twilio-segment]

✅ TVS Validation Passed

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 0b04759

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.59%. Comparing base (99ac8ca) to head (820253f).
⚠️ Report is 1 commits behind head on master.

⚠️ Current head 820253f differs from pull request most recent head 0b04759

Please upload reports for the commit 0b04759 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1391   +/-   ##
=======================================
  Coverage   91.59%   91.59%           
=======================================
  Files         127      127           
  Lines        4142     4142           
  Branches     1033     1033           
=======================================
  Hits         3794     3794           
  Misses        348      348           

Flag	Coverage Δ
browser	92.51% <ø> (ø)
core	90.07% <ø> (ø)
node	89.43% <ø> (ø)

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