doc: add C++20 migration assessment for libModSecurity

[Easton97-Jens]

Motivation

• Provide maintainers a rigorous, security-first decision basis for a possible migration from C++17 to C++20 for libModSecurity.
• Capture repository facts, toolchain/CI realities, ABI/API implications and platform/packaging constraints that make this migration high-risk if done naively.
• Offer concrete, phased migration strategy with Stop/Go criteria and prioritized next steps so maintainers can proceed incrementally and safely.

Description

• Add a new German-language assessment file doc/cpp20-migration-assessment.md containing a 14-section analysis covering Executive Summary, repository/toolchain facts, feature-by-feature C++20 evaluation, detailed risk analysis, CI/tooling readiness, performance/security considerations, effort estimates and a phased migration plan.
• The document includes a C++20 feature matrix (recommend/hold/avoid), concrete refactoring patterns (std::span, std::string_view, std::source_location, targeted constexpr), and 15 prioritized next steps for maintainers.
• This is a documentation-only change and does not modify source, build scripts or CI configuration; it intentionally separates facts from assumptions and prescribes a conservative, toolchain-driven rollout.

Testing

• No code-unit/regression/benchmark tests were run because this is a documentation-only change.
• Presence and contents of the new file were inspected via repository file listing and file-content checks (e.g., nl/sed output) to ensure the assessment was written and placed at doc/cpp20-migration-assessment.md.
• Further validation (CI builds, sanitizer runs, ABI checks and benchmarks) is recommended before any code changes or C++20 flag switch are merged.

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