Remove support for "out-dated" OS(es)

[izzyleung]

This repo provides support for building HAProxy RPM on RHEL 7/8/9, Amazon Linux 1/2/2023, etc.

However, some of those OS(es) have reached their EOL (end of life) phase. I don't believe maintaining support for them is a good idea for the maintainability or the longevity of this project.

• CentOS 7 (the container image of choice for RHEL 7) has reached its EOL phase on June 30, 2024; while it's true that RedHat does provide ELS (Extended Lifecycle Support) until 2029, we should not consider this is an actively supported platform and we should remove support for this OS
• Amazon Linux 1 has reached its EOL phase on Dec 31, 2023, we should consider removing support for this one completely.
  • This also include the benefit of removing any trace of code supporting the ol' SysV Init system 😅

Files can be safely deleted with this change:

• https://github.com/DBezemer/rpm-haproxy/blob/dev/SOURCES/haproxy.syslog.el6
• https://github.com/DBezemer/rpm-haproxy/blob/dev/SOURCES/haproxy.init
• https://github.com/DBezemer/rpm-haproxy/blob/dev/SOURCES/haproxy.syslog.amzn1
• https://github.com/DBezemer/rpm-haproxy/blob/dev/SOURCES/haproxy.syslog.el6
• https://github.com/DBezemer/rpm-haproxy/blob/dev/SOURCES/haproxy.syslog.el7
• https://github.com/DBezemer/rpm-haproxy/blob/dev/Dockerfile7

Files need modifications:

• https://github.com/DBezemer/rpm-haproxy/blob/dev/Makefile
  • Remove mentions of unsupported OS(es)
• https://github.com/DBezemer/rpm-haproxy/blob/dev/SPECS/haproxy.spec
  • Remove mentions of unsupported OS(es), and remove support for SysV Init
• https://github.com/DBezemer/rpm-haproxy/blob/dev/README.md
  • Remove mentions of unsupported OS(es)