[INFRA] Remove C++17 support

Add warning in platform.hpp

include/seqan3/core/platform.hpp

@@ -36,7 +36,11 @@
 
 // C++ standard [required]
 #ifdef __cplusplus
-    static_assert(__cplusplus >= 201703, "SeqAn3 requires C++17, make sure that you have set -std=c++17.");
+#   if (__cplusplus < 201703)
+#       error "SeqAn3 requires C++20, make sure that you have set -std=c++2a (gcc9) or -std=c++20 (gcc10 and higher)."
+#   elif not defined(SEQAN3_DISABLE_CPP17_DIAGNOSTIC) && (__cplusplus >= 201703) && (__cplusplus < 202002)
+#      pragma GCC warning "SeqAn 3.1.x is the last version that supports C++17. Newer SeqAn versions, including this one, might not compile with -std=c++17. To disable this warning, set -std=c++2a (gcc9), -std=c++20 (gcc10 and higher), or -DSEQAN3_DISABLE_CPP17_DIAGNOSTIC."
+#   endif
 #else
 #   error "This is not a C++ compiler."
 #endif
@@ -45,11 +49,6 @@
 #   include <version>
 #endif
 
-// C++ Concepts [required]
-#ifndef __cpp_concepts
-#   error "SeqAn3 requires C++ Concepts, either via the TS (flag: -fconcepts) or via C++20 (flag: -std=c++2a / -std=c++20)."
-#endif
-
 //!\brief Same as writing `{expression} -> concept_name<type1[, ...]>` in a concept definition.
 #if defined(__GNUC__) && (__GNUC__ < 10)
 #   define SEQAN3_RETURN_TYPE_CONSTRAINT(expression, concept_name, ...) \
@@ -59,13 +58,6 @@
        {expression} -> concept_name<__VA_ARGS__>
 #endif
 
-// filesystem [required]
-#if !__has_include(<filesystem>)
-#   if !__has_include(<experimental/filesystem>)
-#      error SeqAn3 requires C++17 filesystem support, but it was not found.
-#   endif
-#endif
-
 // ============================================================================
 //  Dependencies
 // ============================================================================