Deprecate `legend.title.align` and `legend.text.align`

[teunbrand]

This has been alluded to in a 5 year old comment from @clauswilke here.

To recap:

• They're obsolete, as title alignment can now be set using regular theme elements.
• Legend title alignment can currently be set in 4 (!) ways (same story for legend text):
  • theme(legend.title.align = ...)
  • theme(legend.title = element_text(vjust = ..., hjust = ...)
  • guide_legend(title.theme = element_text(hjust = ..., vjust = ...))
  • guide_legend(title.hjust = ..., title.vjust = ...)
• They can only be used to control horizontal alignment and not even vertical alignment.

Arguably, we only need the theme(legend.title = ...) and guide_legend(title.theme = ...) solutions and can deprecate the guide_legend(title.hjust = ..., title.vjust = ...) settings as well as they're also redundant.