Silence Migration Backend Pid in Specs

What does this MR do and why?

Silence the output from Gitlab::Database::Migrations::PgBackendPid in specs.

In migration specs, this code produces output like the following:

All examples were filtered out; ignoring {:focus=>true}

Test environment set up in 4.868654 seconds
main: == [advisory_lock_connection] object_id: 301820, pg_backend_pid: 13359
main: == [advisory_lock_connection] object_id: 301820, pg_backend_pid: 13359
main: == [advisory_lock_connection] object_id: 303180, pg_backend_pid: 13366
main: == [advisory_lock_connection] object_id: 303180, pg_backend_pid: 13366
.main: == [advisory_lock_connection] object_id: 656660, pg_backend_pid: 13367
main: == [advisory_lock_connection] object_id: 656660, pg_backend_pid: 13367
.main: == [advisory_lock_connection] object_id: 934100, pg_backend_pid: 13368
main: == [advisory_lock_connection] object_id: 934100, pg_backend_pid: 13368
.main: == [advisory_lock_connection] object_id: 1233100, pg_backend_pid: 13369
main: == [advisory_lock_connection] object_id: 1233100, pg_backend_pid: 13369
.main: == [advisory_lock_connection] object_id: 1544440, pg_backend_pid: 13370
main: == [advisory_lock_connection] object_id: 1544440, pg_backend_pid: 13370
.main: == [advisory_lock_connection] object_id: 1867100, pg_backend_pid: 13371
main: == [advisory_lock_connection] object_id: 1867100, pg_backend_pid: 13371
.main: == [advisory_lock_connection] object_id: 2383040, pg_backend_pid: 13375
main: == [advisory_lock_connection] object_id: 2383040, pg_backend_pid: 13375
ci: == [advisory_lock_connection] object_id: 2786640, pg_backend_pid: 13379
ci: == [advisory_lock_connection] object_id: 2786640, pg_backend_pid: 13379


Finished in 17.11 seconds (files took 2.54 seconds to load)
6 examples, 0 failures

We disable other output from migrations by setting ActiveRecord::Migration.verbose to false here: https://gitlab.com/gitlab-org/gitlab/-/blob/6768769abb06c1b4d04b50ac4444fc64f7830290/spec/support/helpers/migrations_helpers.rb#L153

So in this MR, if ActiveRecord::Migration.verbose is false, then we also do not print the connection information seen above.

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