chore: remove V1 surface

[bshaffer]

• removed migration_mode support - this bazel option is now ignored, and all generated code is V2
• removed V1 samples gen, v1 client gen, and v1 unit test gen. Removed "V2" from the classname of all V2 classes (as it's now the only version)
• removed V1-specific structures such as GapicEnum and OneOf wrapping classes
• updated all unit and integration tests

[snippet-bot]

Here is the summary of changes.

You are about to delete 5 region tags.

• tests/Unit/ProtoTests/BasicOneofNew/out/samples/BasicOneofNewClient/a_method.php:25, tag basic_generated_BasicOneofNew_AMethod_sync
• tests/Unit/ProtoTests/CustomLroNew/out/samples/CustomLroClient/create_foo.php:25, tag customlro_generated_CustomLro_CreateFoo_sync
• tests/Unit/ProtoTests/CustomLroNew/out/samples/CustomLroOperationsClient/cancel.php:25, tag customlro_generated_CustomLroOperations_Cancel_sync
• tests/Unit/ProtoTests/CustomLroNew/out/samples/CustomLroOperationsClient/delete.php:25, tag customlro_generated_CustomLroOperations_Delete_sync
• tests/Unit/ProtoTests/CustomLroNew/out/samples/CustomLroOperationsClient/get.php:25, tag customlro_generated_CustomLroOperations_Get_sync

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