Feature matrix updates

Hi,

Please see updates for the feature matrix in advance of the PostgreSQL 
15 release:

https://www.postgresql.org/about/featurematrix/

In addition to adding information about new features in PostgreSQL 
15[1][2], this version of the feature matrix adds additional  categories 
including:

* Replication
* JSON
* Transactions and Visibility
* VACUUM and Maintenance
* Client Applications

This is to continue to reduce the amount of features kept in the 
"Backend" section (and to a lesser extent, "Performance"), which 
historically was used as a "catch all".

This also removed the "contrib/" prefix from all of the additional 
modules listed in "Additional Modules".

Please let me know your thoughts on the PG15 bits and if there is 
anything notable that is missing.

Thanks,

Jonathan

[1] https://www.postgresql.org/docs/15/release-15.html
[2] 
https://www.postgresql.org/about/news/postgresql-15-beta-1-released-2453/