docs: Add version-removed for resolve_source

Add a version removed entry to inform users that the `resolve_source`
hook and `default_resolve_source` were removed.

See 179c096

Signed-off-by: Christian Heimes <cheimes@redhat.com>

Author: tiran

docs/reference/hooks.rst

@@ -121,6 +121,8 @@ Resolver hooks
 
 .. currentmodule:: fromager.resolver
 
+.. _resolver_provider_hook:
+
 .. autofromagerhook:: default_resolver_provider
 
     The ``get_resolver_provider()`` function allows an override to change
@@ -191,6 +193,11 @@ Source hooks
 
 .. currentmodule:: fromager.sources
 
+.. versionremoved:: 0.80.0
+   The ``resolve_source`` hook and ``default_resolve_source`` function
+   were removed. Define a :ref:`resolver_provider <resolver_provider_hook>` hook
+   to resolve sources.
+
 .. autofromagerhook:: default_download_source
 
     The ``download_source()`` function is responsible for downloading the