PostgreSQL timestamps are always datetimes

The current behavior is that they are treated as `datetime` normally,
but if they are part of an array, they are treated as `timestamp`. The
only place that seems to be impacted by this is schema dumping, which
shouldn't matter since `t.datetime` and `t.timestamp` are equivalent in
the `PostgreSQL` adapter, anyway.
Let others know why this code is here

[ci skip]
remove ecosystem link that is broken
Merge pull request #15169 from Gaurav2728/gaurav-ecosystem

remove ecosystem link that is broken [ci skip]
Merge pull request #15157 from msgehard/remove_cruft_in_has_secure_password

Remove cruft now that strong_parameters is default [ci skip]
copy edits[ci skip]
A tiny pass through the PostgreSQL guide [ci skip]
Feature detect based on Ruby version.

I didn't want to do this, FNM_EXTGLOB is defined on 2.1.x, but Dir.glob
returns the wrong value on Ruby less than 2.2.0.  Checking for a
case-insensitive FS seems too hard, so just check Ruby version  Checking
for a case-insensitive FS seems too hard, so just check Ruby version.
Merge pull request #15156 from sgrif/sg-postgres-timestamps

PostgreSQL timestamps should always be datetimes