Werk #10030: Still allow working with old customized builtin tag groups

With 1.4 some sample config tag groups have been changed to be "builtin" tag groups. A prominent one is the "agent" tag group. Previous 1.6 releases totally disallowed saving tag configs in case such a tag group was present.

To make the migration easier, the validation is now less strict for the 1.6. It is possible to keep the old builtin tag group until updating to 2.0. With 2.0 these tag groups will not be usable anymore and have to be migrated.

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