Add Trigger_Never to Prepare features check

[damondouglas]

This PR addresses #31461 adding Never Trigger to the list of feature support checks in the Prepare Job context.

To validate this fix:

1. Run Prism server.

./gradlew :runners:prism:runServer

2. Execute :runners:portability:java:ulrLoopbackValidatesRunnerTests on Prism, filtering for org.apache.beam.sdk.testing.PAssertTest:

TEST=org.apache.beam.sdk.testing.PAssertTest
./gradlew :runners:portability:java:ulrLoopbackValidatesRunnerTests -PjobEndpoint=localhost:8073 --tests="$TEST"

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[damondouglas]

Since this is default, does it make sense to remove this?

[lostluck]

Tests removed from this list must be added to another test that expects them to pass. Otherwise we're opening a test regression gap for ourselves down the road.

So, if you're removing this because TestUnimplmented started to fail, then you must move it to the TestImplmenented list down below.

If this test didn't start to fail, then we should leave it here to show we aren't fully done work on TriggerNever.

[damondouglas]

The original ws.GetTrigger() returns a *pipepb.Trigger which would have never passed == &pipepb.Trigger_Default{}. In order for the existing check to evaluate equality would have needed cmp.Equals. Checking the String() was easier and still achieves the desired outcome.

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[lostluck]

Approved after the suggested change.