Improve the security policy

[timvandermeij]

This commit fixes two issues that have been found after commit 2beae7a landed, namely:

• The security checkbox image is not rendering at https://github.com/mozilla/pdf.js/security/policy because the SECURITY.md file is apparently served differently there (because it does work as expected at https://github.com/mozilla/pdf.js/blob/master/.github/SECURITY.md), which causes the relative link not to work. We switch to an absolute link to fix the issue.
• If a security policy is defined it turns out that GitHub automatically adds a row to the "New issue" page; see https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository where it states "When someone creates an issue in your repository, they will see a link to your project's security policy". Since we now have two rows at https://github.com/mozilla/pdf.js/issues/new/choose about the security policy, we remove our own version in favor of the standard GitHub-provided one.