Bump premake to version 5.0.0-beta6

This makes 5.0.0-beta5 the minimally supported version as the gmake2
action was renamed to gmake while gmake was renamed to gmakelegacy. So
from now on gmake action is to be used.

Clear out all old version dependent code.

Fixes: #7789
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

libraries/get-windows-libs.bat

@@ -4,7 +4,7 @@ cd /D "%~dp0"
 
 rem **SVN revision to checkout for windows-libs or windows-libs-amd64**
 rem **Update this line when you commit an update to windows-libs or windows-libs-amd64**
-set "svnrev=28260"
+set "svnrev=28261"
 
 if "%1" == "--amd64" (
     set "LIBS_PATH=win64"

libraries/source/premake-core/build.sh

@@ -8,8 +8,8 @@ set -e
 
 cd "$(dirname "$0")"
 
-PV=5.0.0-beta4
-LIB_VERSION=${PV}+wfg1
+PV=5.0.0-beta6
+LIB_VERSION=${PV}+wfg0
 
 fetch()
 {