security: move security.txt to website from gitpod-io/gitpod

[ghuntley]

currently

1. Accessing https://www.gitpod.io/security or https://www.gitpod.io/.well-known/security.txt is 404
2. Accessing https://gitpod.io/security.txt functions but the hiring link in https://github.com/gitpod-io/gitpod/blob/c8526aeff02a354c1b0a5845bde07b36fa455ae6/components/dashboard/public/.well-known/security.txt is out of date.

changes

• Move security.txt into the website, resolve hiring link, add a handy redirect and resolve 404.
• https://github.com/gitpod-io/gitpod/pull/3459 implements the redirect from https://gitpod.io/security.txt to https://www.gitpod.io/.well-known/security.txt
• Wires up https://docs.github.com/en/github/managing-security-vulnerabilities/adding-a-security-policy-to-your-repository (SECURITY.md)
• Creates GitHub issue template redirect to https://www.gitpod.io/security