Attribute deprecated_readonly

[Zguy]

Adds support for a deprecated_readonly attribute.

While deprecated removes the field entirely from generated code, both getters and setters, deprecated_readonly only removes the setters and keeps the getters.

It's useful for backwards compatibility in some cases. A program might read a buffer written before the deprecation, read the deprecated field if it has a value, do some applicable conversion, and write to a non-deprecated field.

Fixes #7940

Some things to look out for

• I have not fully tested the code generation of all languages. I have tested C++ and C#, the rest I've only visually inspected the generated code of. There might be edge cases I've missed, especially in languages I'm not familiar with.
• I took the liberty of adding the deprecated_readonly attribute to the color field in the monster.fbs sample. Let me know if I should revert this.
• I added deprecated_readonly to the middle of reflection.fbs next to deprecated. I'm not sure if this file needs to keep backwards conformity. If it does, let me know, and I'll move it to the end.
• I wanted to have the C# json serializer support honor the deprecated_readonly attribute. There was no easy way to do this and the solution I came up with I find a bit invasive. I put it in a separate commit for easy removal, if it comes to that.

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[Zguy]

@dbaileychess Not sure what the proper etiquette is here, so apologies if tagging you like this is out of line, but I'd love to get this PR reviewed.

[github-actions]

This pull request is stale because it has been open 6 months with no activity. Please comment or label not-stale, or this will be closed in 14 days.

[Zguy]

I'm still hoping to get this merged. Not stale.

[aardappel]

This looks like a nice change, and clearly some effort went into making it work for all languages, thanks!

My only issue is with the way it is tested, it was added to an existing field in samples. samples are as an example, not for testing. Also better to add a new field for it. So I'd suggest to instead add it to tests/monster_test.fbs as one additional field at the end, e.g. test_desprecated_readonly:bool = true (desprecated_readonly). No buffers will have this field, but that is fine, they can still read the default value.

[aardappel]

Hmm, this PR seems to cause a fuzzing fail, something to do with reflection/binary schemas: https://github.com/google/flatbuffers/actions/runs/16607053932/job/46981960954

It also looks like it is missing changed test code.. though probably easier to first change how it is tested, see above.

There's also Rust failures.