Remove placeholder text from SECURITY.md Supported Versions section

[Copilot]

Addresses review feedback on PR #3 requesting removal of instructional boilerplate from the Supported Versions section.

Changes

• Removed template text "Use this section to tell people about which versions of your project are currently being supported with security updates"
• Replaced example version table (5.1.x, 5.0.x, 4.0.x) with project-appropriate guidance recommending latest version from main branch
• Simplified version table to single "latest" entry

Result

## 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          |
| ------- | ------------------ |
| latest  | :white_check_mark: |

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[LukeLarge]

approve copilot fix

[Copilot]

Pull request overview

This PR removes instructional boilerplate from the SECURITY.md file's Supported Versions section and replaces it with project-specific guidance for tonapi-go.

• Replaced placeholder version table with clear guidance to use the latest version from main branch
• Simplified the version support table to a single "latest" entry with checkmark
• Added explanatory text about security update policy

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.