grpc: switch from master branch to main for upb
grpc: switch from master branch to main for upb

* hardknott and newer branches don't need this as upb repo was removed in:
  commit 15cff67fd6cdb34e3621368fe9ce94a98356f27a
  Author: Anatol Belski <anbelski@linux.microsoft.com>
  Date:   Fri Feb 19 12:39:55 2021 +0000

    grpc: Upgrade 1.24.3 -> 1.35.0

Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>
Signed-off-by: Armin Kuster <525edcd226fb27e7f47fbb319a38fe989c677548@gmail.com>

victronenergy/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded