Build programs as C++20

Use g++-13 for Actions workflows.

Makefile

@@ -1,6 +1,6 @@
 # The binaries that we want to build
 PGMS := bin/lists bin/section_data bin/toc_diff bin/list_issues bin/set_status
-CXXFLAGS := -std=c++17 -Wall -g -O2 -D_GLIBCXX_ASSERTIONS
+CXXFLAGS := -std=c++20 -Wall -g -O2 -D_GLIBCXX_ASSERTIONS
 
 # Running 'make debug' is equivalent to 'make DEBUG=1'
 ifeq "$(MAKECMDGOALS)" "debug"

bin/build_pgms.bat

@@ -1,7 +1,7 @@
 echo "Use -m32 switch to force 32-bit build"
-g++ %* -std=c++17 -DNDEBUG -O2 -o bin/lists.exe  src/date.cpp src/issues.cpp src/status.cpp src/sections.cpp src/mailing_info.cpp src/report_generator.cpp src/metadata.cpp src/lists.cpp
-g++ %* -std=c++17 -o bin/section_data.exe src/section_data.cpp
-g++ %* -std=c++17 -o bin/toc_diff.exe src/toc_diff.cpp
-g++ %* -std=c++17 -DNDEBUG -O2 -o bin/list_issues.exe src/date.cpp src/issues.cpp src/status.cpp src/sections.cpp src/metadata.cpp src/list_issues.cpp
-g++ %* -std=c++17 -DNDEBUG -O2 -o bin/set_status.exe  src/set_status.cpp src/status.cpp
+g++ %* -std=c++20 -DNDEBUG -O2 -o bin/lists.exe  src/date.cpp src/issues.cpp src/status.cpp src/sections.cpp src/mailing_info.cpp src/report_generator.cpp src/metadata.cpp src/lists.cpp
+g++ %* -std=c++20 -o bin/section_data.exe src/section_data.cpp
+g++ %* -std=c++20 -o bin/toc_diff.exe src/toc_diff.cpp
+g++ %* -std=c++20 -DNDEBUG -O2 -o bin/list_issues.exe src/date.cpp src/issues.cpp src/status.cpp src/sections.cpp src/metadata.cpp src/list_issues.cpp
+g++ %* -std=c++20 -DNDEBUG -O2 -o bin/set_status.exe  src/set_status.cpp src/status.cpp