FindBoost: Update MinGW compiler tag for Boost 1.73
Boost 1.73 on MinGW no longer uses the compiler minor version in its library names.
Fixes: #20673 (closed)
Backport: release
Boost 1.73 on MinGW no longer uses the compiler minor version in its library names.
Fixes: #20673 (closed)
Backport: release