Remediation advice in SSRF could be more broadly focused

**Description of the issue**

The [remediation advice for how to mitigate SSRF vulnerabilities ](https://github.com/github/codeql/blob/main/python/ql/src/Security/CWE-918/ServerSideRequestForgery-end.inc.qhelp#L7) is focused on URL allowlisting. While this is fairly good for https schemes where possible to implement, it's not really a comprehensive defense for SSRF.

The advice given assumes that an attacker can't manipulate DNS entries for the domain being allowlisted. It also doesn't offer any advice for mitigating SSRF if an attacker has complete control of the URL and an allowlist isn't practical.

It would be good to add a sentence to the advice to make the remediation advice less specific. Perhaps incorporating a mention of additional network or application controls to prevent servers from making connections to internal resources in the first place (e.g. based on IP addresses).

https://owasp.org/Top10/A10_2021-Server-Side_Request_Forgery_%28SSRF%29/

Beginne mit python/ql/src/Security/CWE-918/ServerSideRequestForgery-end.inc.qhelp bei den im Issue verlinkten Hinweisen zur Behebung und vergleiche sie anschließend mit den OWASP-SSRF-Richtlinien. Erweitere die Hinweise über URL-Allowlisting hinaus, sodass zusätzliche Netzwerk- oder Anwendungskontrollen erwähnt werden, und stelle sicher, dass der endgültige Text sowohl allowlisted als auch von Angreifern kontrollierte URLs behandelt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.