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:
bda450948a

## 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

.github/workflows/docker-test.yml

@@ -54,8 +54,6 @@ jobs:
             image: oraclelinux:8
           - name: oraclelinux-9
             image: oraclelinux:9
-          - name: amazonlinux-2
-            image: amazonlinux:2
           - name: amazonlinux-2023
             image: amazonlinux:2023