Update stale CI references in contributor docs

[ekamran]

Summary

Updates stale contributor documentation that still referred to Travis CI.

Changes

• Replace Travis CI wording with GitHub Actions wording in the code review docs.
• Remove a dead reference to ci/deploy.sh, which no longer exists in wp-cli/wp-cli.
• Update the builds repository link to HTTPS.

Verification

• Confirmed wp-cli/wp-cli no longer has .travis.yml.
• Confirmed wp-cli/wp-cli no longer has ci/deploy.sh.
• Confirmed active GitHub Actions workflows exist in .github/workflows/.
• Ran git diff --check.