refactor(deps): update subxt and address breaking changes

[evilrobot-01]

Summary

As per use-ink/ink-alliance#42, updates subxt to the latest version, addressing breaking changes in the least intrusive way possible.

• [N ] y/n | Does it introduce breaking changes?
• [Y] y/n | Is it dependent on the specific version of ink or pallet-contracts? Yes, it requires a version with the same subxt updates, which is currently available at build(deps): update subxt (and related deps) ink#2504. The dependencies will be updated once that PR is merged to conclude the completion of this PR.

Description

The following breaking changes affected the codebase:

• Removal of substrate-compat feature in https://github.com/paritytech/subxt/releases/tag/v0.39.0. The ink crates now control the version of sp-core/sp-runtime as required. num-traits dependency added for Zero trait access as an example.
• Removal of PairSigner in https://github.com/paritytech/subxt/releases/tag/v0.39.0, along with guidance in https://github.com/paritytech/subxt/blob/d924ece39a5cb369ba5ccde3dc160b5ee006271b/subxt/examples/substrate_compat_signer.rs. This resulted in the biggest breaking changes, which have been implemented in the lightest way possible (i.e. no additional deps introduced unnecessarily).
• Removal of the Hash associated type from the Config trait. Type alias provided to ease the transition.
• Minor changes to the offline signing API and submittable extrinsics/transactions.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[davidsemakula]

LGTM!