Notification about further migration steps I encountered

In response to
> please don't hesitate to notify us about further migration steps you encounter!
> _Source: https://joel-costigliola.github.io/assertj/assertj-swing-migrating.html_

Here are the four additional steps I had to perform when upgrading from `org.easytesting:fest-swing:1.2` to `org.assertj:assertj-swing-junit:3.17.1`:

* `org.fest.swing.driver.JTableCellValidator.validateCellIndices` was renamed to `org.assertj.swing.driver.JTableCellPreconditions.checkCellIndicesInBounds`
* `org.fest.swing.query.ComponentBackgroundQuery.backgroundOf(component)` had to be replaced with `org.assertj.swing.edt.GuiActionRunner.execute(() -> component.getBackground())`
* `org.fest.swing.query.ComponentFontQuery.fontOf(component)` had to be replaced with `org.assertj.swing.edt.GuiActionRunner.execute(() -> component.getFont())`
* `org.fest.swing.query.ComponentForegroundQuery.foregroundOf(component)` had to be replaced with `org.assertj.swing.edt.GuiActionRunner.execute(() -> component.getForeground())`