Merge pull request #18171 from dannon/update-security

[PROCEDURES] Minor adjustments to SECURITY.md to reflect that we call yearly releases LTS

SECURITY.md

@@ -16,8 +16,7 @@ The following branches or releases receive security support:
 
 - Development on the `dev` branch, hosted on GitHub, which will become the next release of Galaxy
 - Releases within the past 12 months.
-  - E.g. 16.04 will receive support until 2017-04. As the month changes to 2017-05 it will become unsupported.
-- There are currently no plans for Long Term Support (LTS) releases.
+  - E.g. 24.0 will receive support for a full year, at which point 25.0 will be available.
 
 For unsupported branches: