Add `master` to `main` branch change in UPDATING.md

[UncleGrumpy]

Adds entry for name change of master branch to main and gives directions for updating local clones of the repo.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later