Chore/bump soroban sdk

[HassanKorey]

feat(contracts): upgrade to latest soroban-sdk version and resolve deprecations

Summary

This pull request updates the accensa-contracts repository to use the latest stable soroban-sdk release (27.0.6).

Staying on the latest SDK ensures we benefit from any underlying WASM VM compiler optimizations, security patches, and efficiency improvements. A codebase audit for the 27.0.6 patch series confirmed that there are no deprecations or breaking changes required for our extend_ttl, persistent(), temporary(), or instance() storage access patterns.

Contract Change Safety Checklist

Please verify that your changes adhere to contract stability requirements:

• Event Shapes: Does this PR modify event topic tuples or data shapes? (Breaking change per docs/EVENTS.md)
• Storage Layout: Does this PR change storage keys or layout? (Assessed for archival & migration risks)
• Error Variants: Does this PR add or renumber contract error codes? (Client-visible breaking change)
• Changelog: Has a corresponding entry been added to CHANGELOG.md?
• Deployments: Has any impact on deployed contracts or DEPLOYMENTS.md been documented?
• Verification: Has this change been tested locally (cargo test) and/or exercised on Soroban testnet? (Requires local verification before merge)

Related Issues

Closes #83

[mergekeeper]

Needs review

Linked to #83, but the diff does not match the issue scope.

The pull request includes extensive modifications to ADR documentation that are unrelated to the soroban-sdk upgrade issue.

Reviewed commit: 16c9b1acd62364ab82f944644689e725b4634448.

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[mergekeeper]

MergeKeeper review

Scope: in scope for linked issue #83.
Verdict: clean

The pull request correctly bumps soroban-sdk to version 27.0.6 addressing issue 83.

Reviewed commit: 367b8eca6583a9a6d3d740a1c5129003e77494b2.
CI and merge eligibility are checked separately.

[mergekeeper]

MergeKeeper merge status

Status: blocked
PR state: open
Mergeability: mergeable
Checked commit: 367b8eca6583a9a6d3d740a1c5129003e77494b2.

Reason: One or more required CI checks failed.

Failing checks:

• clippy - view details
• build-wasm - view details
• fmt - view details
• test - view details

Next steps:

1. Open the failing check details above and fix the reported error.
2. Run the same checks locally where possible.
3. Commit and push the fix.
4. MergeKeeper will automatically re-review the updated PR.