[MISC] Drop GCC9 support.

.github/workflows/ci_linux.yml

@@ -43,12 +43,6 @@ jobs:
             cc: "gcc-10"
             build_type: Release
 
-          - name: "Unit gcc9 (c++2a)"
-            cxx: "g++-9"
-            cc: "gcc-9"
-            build_type: Release
-            cxx_flags: "-std=c++2a"
-
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/ci_misc.yml

@@ -28,14 +28,14 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: "Snippet gcc9"
-            cxx: "g++-9"
-            cc: "gcc-9"
+          - name: "Snippet gcc11"
+            cxx: "g++-11"
+            cc: "gcc-11"
             build: snippet
             build_type: Release
             build_threads: 2
             test_threads: 1 # snippets create and delete files and some separate tests create/delete the same files
-            cxx_flags: "-std=c++2a"
+            cxx_flags: "-std=c++20"
             cmake: 3.10.3
             requires_toolchain: true
             requires_ccache: true
@@ -55,20 +55,6 @@ jobs:
             skip_build_tests: false
             skip_run_tests: false
 
-          - name: "Header gcc9"
-            cxx: "g++-9"
-            cc: "gcc-9"
-            build: header
-            build_type: Debug
-            build_threads: 2
-            test_threads: 2
-            cxx_flags: "-std=c++2a"
-            cmake: 3.10.3
-            requires_toolchain: true
-            requires_ccache: true
-            skip_build_tests: false
-            skip_run_tests: false
-
           - name: "Documentation"
             build: documentation
             build_threads: 2

doc/about/api/index.md

@@ -22,7 +22,7 @@ For details regarding API stability, please see the
 The main requirement for Sharg is that your operating system provides one of the compilers supported by us.
 In general, we only support the latest three major compiler versions.
 We currently support the following compilers on 64-bit operating systems with little-endian CPU architectures:
-  * GCC9, GCC10, GCC11
+  * GCC10, GCC11
 
 \note Only the most recent minor release of a compiler is guaranteed to be supported, e.g. when `gcc-10.4` is released,
 we may drop support for `gcc-10.3`. Since all platforms with an older version receive minor release updates,

include/sharg/concept.hpp

@@ -12,7 +12,7 @@
 
 #pragma once
 
-#include <seqan3/std/concepts>
+#include <concepts>
 
 #include <sharg/enumeration_names.hpp>
 

include/sharg/validators.hpp

@@ -12,12 +12,11 @@
 
 #pragma once
 
+#include <concepts>
 #include <fstream>
+#include <ranges>
 #include <regex>
 
-#include <seqan3/std/concepts>
-#include <seqan3/std/ranges>
-
 #include <sharg/detail/safe_filesystem_entry.hpp>
 #include <sharg/detail/to_string.hpp>
 #include <sharg/exceptions.hpp>