chore(release): consolidate unreleased work into 5.1.0

pub.dev never received 5.1.0 or 5.1.1 — the latest published version is
5.0.0 and the only release tag is core-v5.0.0. The three head sections of
the changelog (Unreleased, 5.1.1, 5.1.0) therefore all described work that
had never shipped, so they are merged into a single 5.1.0 release covering
the Server -> Guild rename, the WebSocket resilience work and the audit
remediation from chantier #458.

Heading levels follow the file's dominant convention (# for the version,
as in 5.0.0 and 4.2.0); the Unreleased and 5.1.1 sections had deviated.

Two prose corrections: the "version deliberately unassigned" preamble is
dropped now that a version is assigned, and "All four sit on code paths
that threw on every invocation" becomes "Several of these" — the list
holds seven entries, and the claim that none broke working consumer code
does not hold for the CommandBuilder and EnvPlaceholder changes.

chore: deploy release 5.0.0

chore: release cache 2.0.0

deploy: prepare release candidate

chore: update dependencies (#321)

* build(pubspec): update dependencies in pubspec.yaml

* refactor: remove unnecessary library declarations from multiple files

* refactor: update function signatures

* fix: specify type for callback parameter in _handleMessage method

deploy: prepare release

fix/add message return type (#241)

* fix: add Message as return type and enforce types

* deploy: prepare pre-release

refactor: remove unused imports

deploy: prepare deployment

deploy: prepare deployment