MINOR: Remove toStruct and fromStruct methods from generated protocol classes

[ijuma]

Update few classes that were still using the removed methods (including
tests that are no longer required).

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[chia7712]

@ijuma Nice cleanup!

[chia7712]

Could we use HIGHEST_SUPPORTED_VERSION and LOWEST_SUPPORTED_VERSION to replace origin condition ("0" and DefaultPrincipalData.SCHEMAS.length) ?

[ijuma]

Good point, addressed.

[chia7712]

the field "ERROR_CODE" is never used.

[chia7712]

There is another invalid description.

You can also deserialize a message from a Struct by calling Message#fromStruct.
The Struct will not be modified.

[dengziming]

This is nice, maybe we will also remove struct.writeTo and struct.read after #9641 #9766 and KAFKA-10863 is accomplished.

[ijuma]

@dengziming Looks like StickyAssignor also uses Struct still. Once all usages are removed, maybe we can delete Struct altogether instead of only some methods in it.

[ijuma]

Hmm, we may need to keep Struct as a type for Schema, however. So you may be right that we can only remove the serialization/deserialization methods.

[dengziming]

Yes, #9766 is trying to replace Struct inStickyAssignor. I am not sure about removing Struct class, I will try to check it, but we can remove serialization/deserialization methods.

[ijuma]

JDK 11 build passed, JDK 8 and 15 had unrelated flaky failures.