aastex version has changed

[Jean-Pierre Chrétien]

Dear developers,

Since TL 2022 at least, aastex version is now 6.31, so aastex631.cls is required 
to run templates and models. That's the file I get with TL 2023.

Looking at the revision history on the AAS site, I see this :

New in AASTeX v6.3
1. New interactive command to highlight interactive figures
2. Changes to collaboration to increase options for truncated author lists
3. New anonymous style to keep the authors, affiliations and acknowledgments 
from showing in the compiled pdf for dual anonymous review
4. Adoption of IAU approved syntax for nominal solar and planetary units
5. The movetableright command to help center wide, portrait tables
6. Updated aasjournal.bst file

New in AASTeX v6.3.1
1. Bug fix in the Anonymous style option
2. Improved equation numbering with the linenumbers option
3. Depreciate the old acknowledgment command
4. De-emphasize the RNAAS style option
5. Added psj command for the Planetary Science Journal

A couple of new options could be inserted in the doc, but currently I will just 
update the version number, knowing that the layout and the exemple and template 
files should be updated also.
I can do this as well if you like, and I can edit the English version of 
Additional.lyx.

-- 
Jean-Pierre