[BEAM-12628] make AvroCoder#useReflectApi default to true

[clairemcginty]

following up on discussion from https://issues.apache.org/jira/browse/BEAM-12628, this changes the default value of AvroCoder#useReflectApi to true. This means that by default, avro string fields will get decoded as CharSequences backed by java.lang.Strings not org.apache.avro.util.Utf8s. This matches Beam's behavior prior to version 2.30.

cc @RyanSkraba , @udim

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[RyanSkraba]

LGTM! This should restore the previous behaviour... Given the discussion on the #15292 this is what more developers that actually use this part of the code will expect!

[RyanSkraba]

If there's going to be an 2.33.0-RC2, it would be really good to include this! It's fixing a really recent regression that touches pipelines in production, and it would be good to restore the expected behavior before new developers get used to the new behavior.

The original PR causing the regression: #14410

R: @aromanenko-dev I think you're up to date on the issue as well!