chore: add version pins for robotframework-browser

With these version pins we can test the new changes in
plone.app.robotframework to add support for playwright based tests (that
uses robotframework-browser as a glue between playwright and
robotframework).

versions.cfg

@@ -185,6 +185,7 @@ zope.sendmail = 5.3
 # but are NOT managed by closely related communities.
 async-generator = 1.10
 attrs = 23.1.0
+backports.cached-property = 1.0.2
 cryptography = 41.0.1
 cssselect = 1.2.0
 decorator = 5.1.1
@@ -204,6 +205,7 @@ Markdown = 3.4.3
 mock = 5.0.2
 orderedmultidict = 1.0.1
 outcome = 1.2.0
+overrides = 7.3.1
 piexif = 1.1.3
 Pillow = 9.5.0
 prompt-toolkit = 2.0.10
@@ -217,6 +219,8 @@ python-dotenv = 1.0.0
 PyYAML = 6.0
 responses = 0.23.1
 robotframework = 6.1
+robotframework-assertion-engine = 1.0.0
+robotframework-browser = 16.2.0
 robotframework-debuglibrary = 2.3.0
 robotframework-pythonlibcore = 4.1.2
 robotframework-selenium2library = 3.0.0