Promote const_generics to a standard feature

[ironcev]

Description

This PR promotes const_generics to a standard feature not anymore behind a feature flag.

#7512 made const_generic experimental feature enabled by default, but still kept it as experimental, with the decision to remove the feature flag as a part of the next breaking release 0.71.0.

• Closes Const Generics #6860.

Breaking change

The const_generic experimental feature is removed. If a compilation used --experimental/no-experimental const_generic or #[cfg(experimental_const_generics)] it will fail now. The solution is to simply remove all the usages of the feature flag.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[cursor]

PR Summary

Medium Risk
Promotes const_generics out of the experimental/flagged path and rewires stdlib encoding/decoding and generic-param handling, which could affect compilation and ABI behavior for arrays/str[N] across projects. Most changes are mechanical removals of cfg(experimental_const_generics) branches, but the surface area is large in sway-lib-std.

Overview
Promotes const_generics to always-on behavior by removing the const_generics entry from sway-features and dropping related #[cfg(experimental_const_generics = ...)] gating in docs and stdlib.

In sway-core, const generic parameters are now always converted into ConstGenericParameters without emitting “feature disabled” diagnostics.

In sway-lib-std, array and str[N] support is simplified to rely on const generics directly: the code generator script stops emitting the many fixed-size non-const-generic AbiEncode/AbiDecode/Debug impls, and the stdlib keeps only the generic [T; N] and str[N] implementations (still respecting experimental_str_array_no_padding).

^{Reviewed by Cursor Bugbot for commit 14da455. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ironcev]

👍

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks

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_{Comparing ironcev/promote-const-generics-feature (14da455) with master (0507c2c)}