Update dependencies

[SplittyDev]

This pull request primarily updates dependencies across the workspace to newer versions and refactors the environment variable cache implementation to use the standard library. The changes improve compatibility, security, and maintainability by aligning with the latest releases and removing an external dependency.

Dependency updates:

• Updated core dependencies in the workspace, including thiserror, uuid, and metrics in Cargo.toml, to their latest major versions for improved compatibility and security.
• Upgraded dependencies in hdrop-db/Cargo.toml, such as diesel, deadpool-diesel, and deadpool, to newer versions for better performance and bug fixes.
• Updated several dependencies in hdrop-server/Cargo.toml, including axum, axum-extra, regex, tower-http, metrics-exporter-prometheus, and tracing libraries, to their latest versions for enhanced features and stability. [1] [2]

Codebase refactoring:

• Replaced usage of the external once_cell crate with the standard library's OnceLock in hdrop-shared/src/env.rs, simplifying dependency management and leveraging the standard library. [1] [2] [3]
• Updated metric registration in hdrop-server/src/server/prometheus_metrics_server.rs to ignore the return values of metric macros, aligning with updated macro signatures in the latest metrics crate.

[Copilot]

Pull request overview

This PR updates multiple dependencies across the workspace to their latest versions and replaces the external once_cell crate with the standard library's OnceLock. It also adds the Zlib license to deny.toml (needed by the new foldhash transitive dependency) and adapts metrics macro calls to align with the updated metrics crate API.

Changes:

• Bumped major and minor versions of workspace and crate-level dependencies (e.g., axum 0.7→0.8, thiserror 1→2, metrics 0.22→0.24, diesel 2.1→2.3, etc.)
• Replaced once_cell::sync::OnceCell with std::sync::OnceLock in hdrop-shared/src/env.rs, removing the once_cell direct dependency
• Updated metrics::gauge!/histogram!/counter! calls to discard return values with _ =, matching the new macro signatures

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Cargo.toml	Bumped workspace deps: thiserror 2, uuid 1.22, metrics 0.24
hdrop-db/Cargo.toml	Bumped diesel 2.3, deadpool-diesel 0.6.1, deadpool 0.12.3
hdrop-server/Cargo.toml	Bumped axum 0.8, axum-extra 0.12, tower-http 0.6, metrics-exporter-prometheus 0.18, etc.
hdrop-shared/Cargo.toml	Removed once_cell dependency
hdrop-shared/src/env.rs	Migrated from OnceCell to OnceLock
hdrop-server/src/server/prometheus_metrics_server.rs	Discarded return values of metrics macros
deny.toml	Added Zlib license for foldhash dependency
Cargo.lock	Updated lockfile to reflect all dependency changes

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