unsupported pg version in ~/.pgrx/config.toml isn't handled

[workingjubilee]

We never actually remove an old version like pg11 from the config.toml, and it's not clear we should. We can tell the user they need to intervene, but we probably should correctly identify when they must. Currently PG_VERSION=all cargo pgrx test doesn't work for my local installation.

Sample busted toml:

[configs]
pg16 = "/home/jubilee/.pgrx/16.0/pgrx-install/bin/pg_config"
pg13 = "/home/jubilee/.pgrx/13.12/pgrx-install/bin/pg_config"
pg12 = "/home/jubilee/.pgrx/12.16/pgrx-install/bin/pg_config"
pg11 = "/home/jubilee/.pgrx/11.21/pgrx-install/bin/pg_config"
pg14 = "/home/jubilee/.pgrx/14.9/pgrx-install/bin/pg_config"
pg15 = "/home/jubilee/.pgrx/15.4/pgrx-install/bin/pg_config"

[workingjubilee]

this really ought to be taken care of before our next release.