bug: cmake -DACTS_BUILD_PLUGIN_ACTSVG=on requires -DCMAKE_CXX_STANDARD=20

```
cmake -B build -S . -DACTS_BUILD_PLUGIN_ACTSVG=on
```
works fine, but if I rerun it on the same build directory, I get:
```
cmake -B build -S . -DACTS_BUILD_PLUGIN_ACTSVG=on

CMake Error at cmake/ActsCompilerOptions.cmake:47 (message):
CMAKE_CXX_STANDARD=17, but ACTS requires C++ >=20
Call Stack (most recent call first):
CMakeLists.txt:227 (include)
```

This can be fixed by adding an explicit `-DCMAKE_CXX_STANDARD=20`. Other CMake options I tried didn't affect this issue. I am using `LCG_108/x86_64-el9-gcc15-opt`, but saw the same issue with previous versions.