Document: Update CONTRIBUTING.md with v1-alpha branch (#7371)
Document: Update CONTRIBUTING.md with v1-alpha branch (#7371)

Change all references of 'next' branch to
'v1-alpha'.

Fix #7370
hai-cea/material-ui,manchesergit/material-ui,hai-cea/material-ui,janmarsicek/material-ui,manchesergit/material-ui,janmarsicek/material-ui,igorbt/material-ui,janmarsicek/material-ui,janmarsicek/material-ui,igorbt/material-ui