Mark minimum version of CBS 7.6.2 for XDCR

[torcolvin]

The release of server 7.6.1 will error as follows:

    xdcr_test.go:44: 
        	Error Trace:	/home/ec2-user/workspace/SyncGateway-Integration/xdcr/xdcr_test.go:44
        	Error:      	Received unexpected error:
        	            	Could not create xdcr cluster: sync_gateway_xdcr. POST /controller/createReplication -> (400) {"errors":{"mobile":"The only allowed values for setting mobile are [Off]"}}
        	Test:       	TestMobileXDCRNoSyncDataCopied

This was not true for pre-release builds. The pre-release 7.6.2 builds allow turning on XDCR.