Fix CRAN NOTEs and failures

[andrjohns]

Addresses the CRAN notes and donttest example failure from #586, and updates the Makevars files for compatibility with rstan 2.26+

[andrjohns]

Diff inflated due to my IDE stripping whitespace, can view with those filtered out here: https://github.com/stan-dev/rstanarm/pull/587/files?diff=unified&w=1

[jgabry]

Thanks @andrjohns!!

[andrjohns]

FYI that I've just found one more issue that will need to be resolved for 2.26+ (functions in the functions block modifying target directly), will update this PR asap

[andrjohns]

That's all the changes from me!

stanc3 doesn't like functions which directly modify target, as well as functions which call other lpdf/lpmf functions without having the lpdf/lpmf suffix themselves. This was causing failures with the stan_functions test under 2.26+. I resolved those issues and expanded the GHA workflow to test against the rstan preview as well.

I've also verified locally that everything compiles and passes under 2.31 as well, so this PR can be considered the minimal changes needed for future compatibility.

Let me know if I've missed or can clarify anything @bgoodri

diff excluding whitespace changes: https://github.com/stan-dev/rstanarm/pull/587/files?diff=unified&w=1

[sambrilleman]

Wow epic stuff @andrjohns - thanks for keeping this thing alive!! 😅