Allow sslrootcert to accept more than one certificate

Hello,

I was looking for a way to provide multiple independent trusted
certificates to the jdbc driver. I came across the parameter sslrootcert.
This parameter is documented as:

"File name of the SSL root certificate. Defaults to defaultdir/root.crt

It can be a PEM encoded X509v3 certificate"

(see https://jdbc.postgresql.org/documentation/head/connect.html#ssl)

The wording indicates that this parameter accepts only one single
certificate in defauldir/root.crt (or the file to which sslrootcert points
to). However, according to the code which is used to build the java
KeyStore it accepts more than one certificate (as LibPQFactory calls
java.security.cert.CertificateFactory.generateCertificates(InputStream)).
Similar to the sslrootcert parameter for libpq the file may contain
multiple (even independent, i.e. they don't need to build a single chain)
concatenated PEM encoded certificates.

The documentation for sslrootcert in libpq states:

"
This parameter specifies the name of a file containing SSL certificate
authority (CA) certificate(s). If the file exists, the server's certificate
will be verified to be signed by one of these authorities. The default is
~/.postgresql/root.crt
"

In my opinion adapting the wording in the documentation for sslrootcert in
the jdbc driver documentation and allow multiple certificates stored in the
file to which sslrootcert points to makes it also more compatible with
sslrootcert of libpq.

If my understanding is correct I'm happy to enter an issue or open a PR

Regards,

Christian Schaefer