fix: python3 compatibility and rocky8 packaging

[hussainbani]

This pull request updates the build and packaging system to move from CentOS 7 to Rocky Linux 8 (a CentOS 8 successor), modernizes the Docker and build scripts, and improves compatibility with newer Ruby and Python versions. These changes ensure continued support and easier maintenance by using more up-to-date base images and tools. The most important changes are grouped below:

Migration to Rocky Linux 8 and Modernization of Docker Build:

• Replaced the old Dockerfile-centos-7 (CentOS 7) with a new Dockerfile-centos based on rockylinux:8, updated to use Ruby 3 and newer FPM and Python versions, and simplified the build steps. [1] [2]
• Updated the Makefile to use the new Dockerfile-centos for CentOS builds.
• Changed default environment variables to target CentOS/Rocky 8 and FPM 1.16.0 instead of CentOS 7 and FPM 1.11.0 in build-linux/make_common.env.

Packaging Script Improvements:

• Updated build-linux/make_package.sh to use the system Ruby and FPM for Rocky 8+, and added a check to warn if GPG signing keys are not set up, building unsigned packages in that case. [1] [2]
• Improved build-linux/make_rpm.sh to create required directories if they do not exist, making the script more robust.

Python Path Handling and Versioning:

• Simplified and unified logic for determining the Python version and installation path in RPM post-install and pre-remove scripts, ensuring compatibility with Python 3. [1] [2]
• Bumped the package version in pdagent/__init__.py from 1.7.4 to 1.8.0.