feat: drop Amazon Linux 2 support (#1332)

## Summary

- Remove Amazon Linux 2 support from the installer
- Amazon Linux 2023 remains fully supported

## Motivation

Amazon Linux 2 is reaching EOL.

Additionally, Amazon Linux 2 ships with **OpenSSL 1.0.2k** (from 2017)
which is incompatible with Easy-RSA 3.2.x. The newer Easy-RSA versions
use `openssl x509 -ext` which doesn't exist in OpenSSL 1.0.x, causing
certificate generation to fail.

This blocks our ability to upgrade Easy-RSA:
https://github.com/angristan/openvpn-install/commit/bda450948a933224f4f779a24a44c6279e1574a1

## Changes

- Updated OS detection to reject Amazon Linux 2 with a clear message
- Removed Amazon Linux 2 specific code paths (EPEL installation, yum
commands)
- Removed from CI test matrix
- Updated README supported distributions table
- Updated Makefile test targets
- Also, add Amazon Linux 2023 Unbound handling

README.md

@@ -142,7 +142,6 @@ The script supports these Linux distributions:
 |                    | Support |
 | ------------------ | ------- |
 | AlmaLinux >= 8     | ✅ 🤖   |
-| Amazon Linux 2     | ✅ 🤖   |
 | Amazon Linux 2023  | ✅ 🤖   |
 | Arch Linux         | ✅ 🤖   |
 | CentOS Stream >= 8 | ✅ 🤖   |