feat: upgrade casbin to v3 and bump module path to v5

[shushenghong]

Summary

Upgrade the adapter from casbin/v2 to casbin/v3, and bump the module path from v4 to v5 accordingly.

Why bump the major version

casbin v3 is a breaking-change major release:

• Imports move from github.com/casbin/casbin/v2/... to v3/.... Since casbin/v2/persist.Adapter and casbin/v3/persist.Adapter are distinct types in distinct packages (Go has no structural interface inheritance across module versions), an adapter built against v2 cannot satisfy v3's persist.Adapter interface and vice versa.
• Several Enforcer methods (GetPolicy, HasPolicy, GetAllRoles, GetAllSubjects, ...) gained an error return value, breaking direct callers.

By Go module semantic import versioning, downstream users importing the v2-based adapter (mongodb-adapter/v4) cannot transparently consume a v3-based adapter at the same import path. Therefore this adapter must also bump its major version: v4 → v5.

Changes

• go.mod: module path mongodb-adapter/v4 → v5; casbin/v2 v2.71.1 → casbin/v3 v3.10.0; refresh transitive deps (drop Knetic/govaluate, add bmatcuk/doublestar/v4, casbin/govaluate v1.3.0, google/uuid).
• adapter.go: switch model / persist imports from casbin/v2 to v3. The Adapter / BatchAdapter / FilteredAdapter / UpdatableAdapter interface signatures themselves are otherwise unchanged, so the adapter implementation needed no logic changes.
• adapter_test.go: switch casbin and util imports to v3; adapt testGetPolicy / testGetPolicyWithoutOrder helpers to the new GetPolicy() ([][]string, error) signature.
• README.md: bump install command and example imports to v5 / casbin/v3.

Validation

• go build ./... and go vet ./... pass.
• go test -count=1 -run='^$' ./... (compile-only) passes.
• Full integration tests still rely on a live MongoDB instance via TEST_MONGODB_URL / TEST_REPLICA_SET_URL; the existing .github/workflows/ci.yml should run them unchanged on the v3 codebase.

Downstream context

This unblocks gorm-adapter/v3 users who already run on casbin v3 but have to keep an indirect casbin/v2 dependency only because of this adapter (e.g. https://github.com/gun-go/sgo and similar frameworks dual-supporting SQL + Mongo policy storage end up shipping both v2 and v3 in the same binary today).

Test plan

• CI: confirm go test -v ./... passes against the bundled MongoDB / replica-set services.
• Maintainer review of the major-version bump and release process (semantic-release config will need to consider that v5 lives at this same repo path going forward).