Update build yamls for new preview version (#843)

Author: anamnavi

.pipelines/PowerShell-Docker-Image-Build-Official.yml

@@ -145,7 +145,7 @@ extends:
       - template: /.vsts-ci/previewReleaseStage.yml@self
         parameters:
           channel: ${{ parameters.releaseChannel }}
-          channelPath: 'release/7-5'
+          channelPath: 'release/7-6'
     - ${{ if eq(parameters.releaseChannel, 'lts') }}:
       - template: /.vsts-ci/ltsReleaseStage.yml@self
         parameters: