docs: Update security policy

Author: eemeli

docs/SECURITY.md

@@ -1,14 +1,16 @@
 # Security Policy
 
 Security updates will be provided for the most recent minor v1 and v2 releases of `yaml`.
+Security updates for prerelease versions are not provided.
 
 No updates are provided for earlier releases.
+After a 3.0.0 release is published, no updates will be provided for v1 releases.
 
 To report a suspected security vulnerability,
 please do so on GitHub via this form:
 https://github.com/eemeli/yaml/security/advisories/new
 
-You may expect a response within 48 hours.
+You may expect a response within one week.
 
-For extended support or a stronger guarantee,
+For extended support, a stronger guarantee, or a faster response time,
 please reach out to the maintainer to discuss a support agreement.