Remove --process-dependency-links from UPGRADE.rst (#4485)

* Remove --process-dependency-links from UPGRADE.rst

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

UPGRADE.rst

@@ -18,7 +18,7 @@ instructions that may be required are listed later in this document.
 
    .. code:: bash
 
-       pip install --upgrade --process-dependency-links matrix-synapse
+       pip install --upgrade matrix-synapse
 
        # restart synapse
        synctl restart

changelog.d/4485.misc

@@ -0,0 +1 @@
+Remove deprecated --process-dependency-links option from UPGRADE.rst