docs: clarify SECURITY.md version support table with specific ranges and EOL dates (#8861)

* docs: clarify SECURITY.md version support table with specific ranges and EOL dates

- Add detailed version support table with 3.x (actively supported), 2.x (security fixes until Dec 31, 2026), and 1.x (EOL since Jan 1, 2024)
- Define what each support level means for users
- Add migration guidance for users on older versions
- Replace vague version ranges with specific, actionable information

Signed-off-by: localai-bot <localai-bot@noreply.github.com>

* Apply suggestions from code review

Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

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Signed-off-by: localai-bot <localai-bot@noreply.github.com>
Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@noreply.github.com>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>