Update Boot2Qt labels to have MinGW instead of MSVC

Since the MinGW is apparently more suitable for Boot2Qt, update labels from msvc to mingw, in some early stage of release, e.g. for 6.8.0 during FF phase, before beta1.

Common syntax with the configuration strings is to separate host from target with _, Boot2Qt labels usually use - which is used to separate items inside of the host/target definition, maybe that could be changed also for 6.8 when doing the renaming to MinGW, e.g:
win-msvc2019-Windows10-22H2-x64-imx8mm-lpddr4-evk ->

win-MinGW13.1.0-Windows10-22H2-x64_imx8mm-lpddr4-evk