Use only MinGW VSS

[tuxmaster5000]

Fix #1843

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[arogge]

This seems to work pretty well so far. Thank you!

There are three things that should be addressed before we can merge this pull request.

rename the header to vss_client.h

calling it vss_client.h would align its name with its content, which is mainly class VssClient.

fix include guard in header

the include guard (for the renamed header) should be BAREOS_WIN32_INCLUDE_VSS_CLIENT_H_

update copyright years in changed files

the copyright header of the files you changed still states the latest update was 2023. Even for minor changes the year needs to be updated.

[sebsura]

Looks good to me. Thanks for your work!