build: branch manager should use "7.3.x" as patch branch (#15097)
build: branch manager should use "7.3.x" as patch branch (#15097)

Updates the branch manager to validate patch PRs against the
`7.3.x` branch. This is necessary because currently some PRs
are marked as failing because they no longer apply to the
outdated `7.2.x` branch.
andrewseguin/material2,angular/components,Jefiozie/material2,mmalerba/material2,josephperrott/material2,DevVersion/material2,DevVersion/material2,mmalerba/material2,josephperrott/material2,josephperrott/material2,andrewseguin/material2,josephperrott/material2,DevVersion/material2,angular/components,andrewseguin/material2,angular/components,jelbourn/material2,josephperrott/material2,mmalerba/material2,DevVersion/material2,jelbourn/material2,Jefiozie/material2,crisbeto/material2,Jefiozie/material2,crisbeto/material2,jelbourn/material2,Jefiozie/material2,crisbeto/material2,Jefiozie/material2,jelbourn/material2,mmalerba/material2,andrewseguin/material2,angular/components,jelbourn/material2,mmalerba/material2,DevVersion/material2,andrewseguin/material2,crisbeto/material2