lang: Deprecate #[interface] attribute

[acheroncrypto]

Problem

Adding support for custom discriminators made the #[interface] attribute redundant, since the only thing this attribute does is to override the instruction's discriminator. For example:

anchor/tests/spl/transfer-hook/programs/transfer-hook/src/lib.rs

Line 71 in e6d7daf

#[interface(spl_transfer_hook_interface::execute)]

can now be replaced with:

#[instruction(discriminator = ExecuteInstruction::SPL_DISCRIMINATOR_SLICE)]

Many ways to achieve the same thing often leads to confusion for users, and also extra bloat in our codebase.

Summary of changes

Deprecate the #[interface] attribute and add a deprecation note explaining how to migrate.

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Note: This PR is part of a greater effort explained in #3097.

[vercel]

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