Add version pins for robotframework-browser

[gforcada]

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).

That's part of #868 but it's the minimal changes needed so we can get https://github.com/plone/plone.app.robotframework/pull/149 to run.

[gforcada]

@jenkins-plone-org please run jobs

[gforcada]

@mauritsvanrees I'm merging it, so we can unblock https://github.com/plone/plone.app.robotframework/pull/149 in jenkins, it will fail, but not at running buildout 😅