[SVLS-8070] remove serverless-init CI from lambda extension repo

Serverless-init CI/CD has been extracted to its own repo (serverless-init-ci).
Remove the old workflows, Dockerfiles, build scripts, and local tests that are
now redundant. Keep publish-serverless-init-to-ghcr.yml and the chainguard STS
policy since the new GitLab pipeline triggers them for GHCR image publishing.

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