[dev]: utilize GitHub security tools

[jsjoeio]

@jawnsy brought up adding dependabot to code-server in #2830

I took a look at the Security tab and it looks like there is a lot of security tools provided by GitHub that we should be utilizing (including dependabot alerts).

[jsjoeio]

Now that we're using CodeQL (#3229) and trivy (#3261) and dependabot (#2830), we can close this.