[ci] set MinimumVisualStudioVersion to MSVS 2015 (#6689)

Author: StrikerRUS

windows/LightGBM.sln

@@ -1,7 +1,7 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 14
 VisualStudioVersion = 14.0.25420.1
-MinimumVisualStudioVersion = 10.0.40219.1
+MinimumVisualStudioVersion = 14.0.23107.0
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "LightGBM", "LightGBM.vcxproj", "{F31C0B5D-715E-4953-AA1B-8D2AEEE4344C}"
 EndProject
 Global