V8 Testbench browser version recommendations are very outdated

[Ansku]

E.g. the Known Issues page still recommends Firefox ESR 45 or 47. The Tutorial and Quickstart both recommend Firefox ESR 45. None of these pages have got any significant updates since 2017.

Given that Vaadin 8 has already reached End of Life, chances are very few people care about the Tutorial or the Quickstart at this point, but since we still make extended maintenance releases of Vaadin 8 TestBench we could perhaps at least add some warning about the Known Issues being particularly outdated.