Remove placeholder text from SECURITY.md Supported Versions section (#4)

* Initial plan

* Remove placeholder text from SECURITY.md and update with project-specific content

Co-authored-by: LukeLarge <162439255+LukeLarge@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: LukeLarge <162439255+LukeLarge@users.noreply.github.com>

Author: Copilot

SECURITY.md

@@ -2,13 +2,11 @@
 
 ## Supported Versions
 
+We recommend always using the latest version of tonapi-go from the main branch. Security updates will be applied to the main branch.
 
 | Version | Supported          |
 | ------- | ------------------ |
-| 5.1.x   | :white_check_mark: |
-| 5.0.x   | :x:                |
-| 4.0.x   | :white_check_mark: |
-| < 4.0   | :x:                |
+| latest  | :white_check_mark: |
 
 ## Reporting a Vulnerability