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

Makefile

@@ -102,9 +102,6 @@ test-oracle-8:
 test-oracle-9:
 	$(MAKE) test BASE_IMAGE=oraclelinux:9
 
-test-amazon-2:
-	$(MAKE) test BASE_IMAGE=amazonlinux:2
-
 test-amazon-2023:
 	$(MAKE) test BASE_IMAGE=amazonlinux:2023
 
@@ -130,7 +127,6 @@ test-all:
 	$(MAKE) test-almalinux-9
 	$(MAKE) test-oracle-8
 	$(MAKE) test-oracle-9
-	$(MAKE) test-amazon-2
 	$(MAKE) test-amazon-2023
 	$(MAKE) test-arch
 	$(MAKE) test-centos-stream-9