Update Image block styles and captions

There are several actions to be taken for the Image block once me upgrade to WP 5.6:

• Remove caption formatting options: it will be possible to make the caption bold/italic/etc and we don't want to allow that, for consistency (see screenshot)
• Remove "Rounded" style (see screenshot) and therefore the whole "Styles" section which is no longer needed since there will be only one left - done in this PR
• Fix caption alignment option in the sidebar (from P4 options) which now doesn't update properly, probably due to this line taking the previous value of "captionAlignment" (also the buttons to change the alignment look weird on hover, that should be fixed - done in this PR)