setup: bump oauthclient major version

* Bumping the major version of invenio-oauthclient to 6.0.0

* This includes
inveniosoftware/invenio-oauthclient#360 which is
not necessarily breaking but requires a manual DB migration for very
large instances, as documented in the module's upgrade guide.

* Further changes are yet come in v6, such as
inveniosoftware/invenio-oauthclient#328.

Author: palkerecsenyi

setup.cfg

@@ -42,7 +42,7 @@ install_requires =
     invenio-access>=4.1.0,<5.0.0
     invenio-accounts>=6.1.1,<7.0.0
     invenio-oauth2server>=3.2.0,<4.0.0
-    invenio-oauthclient>=5.1.0,<6.0.0
+    invenio-oauthclient>=6.0.0,<7.0.0
     invenio-userprofiles>=4.0.0,<5.0.0
     invenio-indexer>=3.1.0,<4.0.0
     invenio-jsonschemas>=2.1.0,<3.0.0