Bump cxx standard to 17.

[arekm]

Fixes building with libicu 76 which relies on standard 17.

(libicu 76.1 is part of upcoming Debian Trixie)

With 11:

[ 39%] Building CXX object src/lib/CMakeFiles/digiham_util.dir/charset.cpp.o
cd /home/sdr/test/owrx-build/aarch64/digiham/obj-aarch64-linux-gnu/src/lib && /usr/bin/c++ -DVERSION="0.6.2" -I/home/sdr/test/owrx-build/aarch64/digiham/include -I/home/sdr/test/owrx-build/aarch64/digiham/src/lib -g -O2 -ffile-prefix-map=/home/sdr/test/owrx-build/aarch64/digiham=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -mbranch-protection=standard -Wdate-time -D_FORTIFY_SOURCE=2 -std=gnu++11 -fPIC -MD -MT src/lib/CMakeFiles/digiham_util.dir/charset.cpp.o -MF CMakeFiles/digiham_util.dir/charset.cpp.o.d -o CMakeFiles/digiham_util.dir/charset.cpp.o -c /home/sdr/test/owrx-build/aarch64/digiham/src/lib/charset.cpp
In file included from /usr/include/unicode/uenum.h:25,
from /usr/include/unicode/ucnv.h:52,
from /home/sdr/test/owrx-build/aarch64/digiham/src/lib/charset.cpp:3:
/usr/include/unicode/localpointer.h:561:26: error: parameter declared ‘auto’
561 | template <typename Type, auto closeFunction>
| ^~~~
/usr/include/unicode/localpointer.h:573:76: error: template argument 2 is invalid
573 | explicit LocalOpenPointer(std::unique_ptr<Type, decltype(closeFunction)> &&p)
| ^
/usr/include/unicode/localpointer.h:583:78: error: template argument 2 is invalid
583 | LocalOpenPointer &operator=(std::unique_ptr<Type, decltype(closeFunction)> &&p) {
| ^
/usr/include/unicode/localpointer.h:599:59: error: template argument 2 is invalid
599 | operator std::unique_ptr<Type, decltype(closeFunction)> () && {
| ^

[jketterl]

Thank you for the pull request.

I am a bit concerned about the backwards compatibility... Do you happen to know how far back this would still work in terms of Debian releases? I'd like the code to be able to compile on stable, so ideally this should still work on bullseye.