[SVLS-8070] Remove serverless-init CI from lambda extension repo (#1163)

## Summary
- Serverless-init CI/CD has been extracted to its own repo
([serverless-init-ci](https://gitlab.ddbuild.io/DataDog/serverless-init-ci))
- Remove old workflows (`release-serverless-init`,
`nightly-serverless-init`, `serverless-init-vulnerability-scan`,
`test-qemu-versions`), Dockerfiles, build scripts, and local tests that
are now redundant
- Keep `publish-serverless-init-to-ghcr.yml` and the chainguard STS
policy — the new GitLab pipeline triggers these for GHCR image
publishing. Remove them later when ghcr use drops to zero.
- Clean up `build_binary_and_layer_dockerized.sh` to remove
`SERVERLESS_INIT` conditional branches
- Fix CODEOWNERS typo (wrong extension and path for the GHCR publish
workflow)

**Do not merge** until the first serverless-init release through the new
CI pipeline is confirmed working.

## Test plan
- [ ] Verify no lambda-extension CI workflows are broken
- [X] Confirm first serverless-init release via new serverless-init-ci
pipeline succeeds
- [X] Confirm GHCR publish still works (triggered from new pipeline →
`publish-serverless-init-to-ghcr.yml`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Lewis-E

.github/CODEOWNERS

@@ -1,4 +1,4 @@
 * @DataDog/serverless-aws
 
 .github/chainguard/serverless-init-ci-publish.sts.yaml @DataDog/serverless
-.github/publish-serverless-init-to-ghcr.yaml @DataDog/serverless
+.github/workflows/publish-serverless-init-to-ghcr.yml @DataDog/serverless