Improve security reports

- Remove dummy issue template since GitHub now presents security
policies.
- Add version and operating system information to SECURITY.md
- Use forum instead of email (this was discussed when I created the
  security report issue template).

.github/SECURITY.md

@@ -1,8 +1,32 @@
 # Security Policy
 
+## Supported Versions
+
+Only the latest version of Multi Theft Auto is supported with security updates.
+
+| Version            | Supported          |
+| ------------------ | ------------------ |
+| 1.5.6              | :white_check_mark: |
+| 1.5.6 (xp/vista)   | :white_check_mark: |
+| < 1.5.6            | :x:                |
+
+Please consider upgrading to the latest version of Windows if you discover
+security problems with Windows XP or Vista. Since April 2014 and April 2017, Microsoft
+no longer provides security updates for both these operating systems, respectively.
+
+1.5.7 will be the final release of Multi Theft Auto that supports
+Windows XP and Windows Vista.
+
 ## Reporting a Vulnerability
 
-Please submit your cheats or security vulnerabilities to @ccw or @Dutchman101 on forum.mtasa.com. Please do not submit them here.
+Before disclosing a security vulnerability, please try to confirm
+that they work with the latest version of Multi Theft Auto.
+
+Please submit your security vulnerabilities to [@ccw] on forum.mtasa.com.
+
+Cheats can also be submitted to [@ccw] or [@Dutchman101] on forum.mtasa.com.
+
+Do not submit them publicly, on GitHub or anywhere else.
 
-@ccw can be found here: https://forum.mtasa.com/profile/7264-ccw/ or send email to ccw808@gmail.com
-@Dutchman101 can be found here: https://forum.mtasa.com/profile/38052-dutchman101/ or send email to mcak105@outlook.com
+[@ccw]: https://forum.mtasa.com/profile/7264-ccw/
+[@Dutchman101]: https://forum.mtasa.com/profile/38052-dutchman101/