Update semgrep.yml (#245)

Updating Semgrep.yml file - Semgrep is a tool that will be used to scan
Cloudflare's public repos for Supply chain, code and secrets. This work
is part of Application & Product Security team's initiative to onboard
Semgrep onto all of Cloudflare's public repos.

In case of any questions, please reach out to "Hrushikesh Deshpande" on
cf internal chat.

.github/workflows/semgrep.yml

@@ -1,4 +1,3 @@
-
 on:
   pull_request: {}
   workflow_dispatch: {}
@@ -12,14 +11,14 @@ name: Semgrep config
 jobs:
   semgrep:
     name: semgrep/ci
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
       SEMGREP_URL: https://cloudflare.semgrep.dev
       SEMGREP_APP_URL: https://cloudflare.semgrep.dev
       SEMGREP_VERSION_CHECK_URL: https://cloudflare.semgrep.dev/api/check-version
     container:
-      image: returntocorp/semgrep
+      image: semgrep/semgrep
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: semgrep ci