cloud-init: switch branch to main

Cloud-init has removed the master branch, and has switched
to using main.

So we adjust our recipe accordingly to avoid fetcher errors.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

recipes-extended/cloud-init/cloud-init_20.3.bb

@@ -5,7 +5,7 @@ LICENSE = "GPLv3"
 LIC_FILES_CHKSUM = "file://LICENSE;md5=c6dd79b6ec2130a3364f6fa9d6380408"
 
 SRCREV = "1431c8a1bddaabf85e1bbb32bf316a3aef20036e"
-SRC_URI = "git://github.com/canonical/cloud-init;branch=master \
+SRC_URI = "git://github.com/canonical/cloud-init;branch=main;protocol=https \
     file://cloud-init-source-local-lsb-functions.patch \
     file://0001-setup.py-check-for-install-anywhere-in-args.patch \
     file://0001-setup.py-respect-udevdir-variable.patch \