Remove PostgreSQL 9.6 and 10 from the CI

[juliogonzalez]

While tds_fdw should still work fine with PostgreSQL 9.6 and 10, we are removing this PostgreSQL version from the CI, as it's end of life: https://www.postgresql.org/support/versioning/

Since it's end of life, the PostgreSQL repositories from the PostgreSQL Development Group we use, do not offer 9.6 anymore for RHEL and clones:

• https://download.postgresql.org/pub/repos/yum/

PostgresSQL 10 is still there, but it will probably be removed soon.

And while we could still test Deb packages (https://download.postgresql.org/pub/repos/apt/pool/), that would force specific restrictions on the CI matrix and, again, 9.6 is EoL.

[juliogonzalez]

I just noticed PostgreSQL 10 is also EoL, so I will change the PR.

[jenkins-juliogonzalez]

Test PASSed.

[juliogonzalez]

Merging, as we don't have 9.6 or 10 results anymore at https://jenkins.juliogonzalez.es/job/tds_fdw-build/