chore: pin third-party GitHub Actions to commit SHAs

[mahangu]

Pins third-party GitHub Actions in this repo to immutable commit SHAs.

This is a draft PR for review before merging. It was prepared with agent assistance and manually verified.

Tracking: DEVPROD-1072

Repo-level summary:

• Pinned distinct third-party action refs in this PR: 2
• Repo-level unpinned usage count from the trunk recheck: 6
• Dependabot GitHub Actions coverage: created (.github/dependabot.yml)

Verification commands:

# shivammathur/setup-php # 2.37.1 -> 7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc
gh api repos/shivammathur/setup-php/commits/2.37.1 --jq '.sha'
# expected: 7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc

# slackapi/slack-github-action # v1.24.0 -> e28cf165c92ffef168d23c5c9000cffc8a25e117
gh api repos/slackapi/slack-github-action/commits/v1.24.0 --jq '.sha'
# expected: e28cf165c92ffef168d23c5c9000cffc8a25e117

[coveralls]

coverage: 80.271% (+0.02%) from 80.255% — chore/devprod-1072-pin-actions-shas into main

[markbiek]

LGTM!