Remove deprecated GenesisBuild trait.

[Kanasjnr]

Summary

• Removed the deprecated GenesisBuild trait from frame_support::traits
• Dropped the re-export and macro support for GenesisBuild<T> / GenesisBuild<T, I>
• Pallet #[pallet::genesis_build] now only accepts BuildGenesisConfig
• Updated docs and UI test expectations accordingly

Part of #11561

Migration

If you still use the old genesis build syntax:

// Before
impl<T: Config> GenesisBuild<T> for GenesisConfig<T> { ... }

// After
impl<T: Config> BuildGenesisConfig for GenesisConfig<T> { ... }

BuildStorage / assimilate_storage for genesis configs is still generated by the pallet macro only the trait name changes.

[Kanasjnr]

/cmd prdoc --audience runtime_dev --bump major

[Kanasjnr]

/cmd label T13-deprecation

[bkontur]

@Kanasjnr please check https://github.com/paritytech/polkadot-sdk/actions/runs/28531804382/job/84598850036?pr=12529#step:4:878