Change c++ standard to c++23

[SiskaPavel]

Summary

Change c++ standard from c++20 to c++23 to support new features.

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✅ Checks

• Relevant documentation was updated (if needed)
• CI pipeline passes (build, tests, lint, static analysis)
• New code follows clang-tidy rules
• The change is clearly related to a specific issue / requirement
• Documentation (e.g. Doxygen) is updated if needed
• The PR is rebased on the latest master
• No unrelated changes are included

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[Copilot]

Pull Request Overview

This PR upgrades the C++ standard from C++20 to C++23 to enable the use of newer language features. The changes update both the CMake configuration and RPM package specifications to ensure consistent C++23 support across all build environments.

• Updates CMAKE_CXX_STANDARD from 20 to 23 in the main build configuration
• Simplifies GCC toolset requirements in RPM specs by consolidating version checks
• Removes redundant GCC version specifications for RHEL 9+ systems

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
CMakeLists.txt	Updates C++ standard requirement from C++20 to C++23
pkg/rpm/ipfixprobe.spec.in	Consolidates GCC toolset requirements using unified version check
pkg/rpm/ipfixprobe-nemea.spec.in	Consolidates GCC toolset requirements using unified version check
pkg/rpm/ipfixprobe-msec.spec.in	Consolidates GCC toolset requirements using unified version check

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