fix: bump core deps to v4 and remove EasyJSON support

[aaron-zeisler]

Summary

Updates the root go-server-sdk/v7 module to use the v4 releases of core dependencies, completing the EasyJSON removal for this library.

• go-jsonstream /v3 → /v4 (v4.0.0)
• go-sdk-common /v3 → /v4 (v4.0.0)
• go-server-sdk-evaluation /v3 → /v4 (v4.0.0)
• go-sdk-events v3.5.0 → v3.6.1
• Remove EasyJSON section from README
• All /v3 imports rewritten to /v4

Dependency Chain (v4 cascade)

go-jsonstream/v4        <- merged, v4.0.0 released
└─ go-sdk-common/v4     <- merged, v4.0.0 released
   ├─ go-server-sdk-evaluation/v4 <- merged, v4.0.0 released
   ├─ go-sdk-events/v3  <- merged, v3.6.1 released
   └─ go-server-sdk/v7 (this PR) <- root module only

Sequence of events

This PR is intentionally scoped to unblock the v4 cascade without getting stuck on circular dependencies between the root module and its sub-modules:

1. Only the root module of go-server-sdk has been updated to v4 dependencies and /v4 imports. Sub-modules are handled separately (see below).

2. ldai and ldotel will be updated in a separate pull request (#382) due to circular dependencies between those sub-modules and the root module. They should merge after this PR lands.

3. testservice is temporarily configured with pseudo-versions of the integration modules (consul, dynamodb, redis-go-redis) so contract-test CI can compile against v4-capable deps via replace go-server-sdk => ../. Those pseudo-versions will be replaced with real tagged releases in a subsequent pull request (Stage 5 cleanup).

Scope

• In this PR: root module + testservice (pseudo-version pins only)
• Separate PR: ldai, ldotel (#382)
• Follow-up PR: flip testservice pseudo-versions to real releases; ldmiddleware if needed

Test plan

• go build ./... clean (root module)
• make build-contract-tests clean (testservice with integration pseudo-versions)
• go mod tidy produces no diff
• No /v3 imports of bumped libs remaining in root module or testservice

Context

Part of SDK-2113 (EasyJSON removal epic). Tracks SDK-2119.

via LD Research 🤖

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Note

Medium Risk
Wide import and dependency churn touches evaluation, JSON streaming, and contract-test wiring; behavior should be unchanged but regressions could show up in flag evaluation or integration tests until sibling modules (ldai, ldotel) and real integration tags land.

Overview
This PR moves the root go-server-sdk/v7 module onto the v4 LaunchDarkly core libraries and drops optional EasyJSON integration from the product surface.

Dependencies: go.mod / go.sum now require go-sdk-common/v4, go-jsonstream/v4, and go-server-sdk-evaluation/v4, with go-sdk-events bumped to v3.6.1. Indirect mailru/easyjson and josharian/intern entries are removed from the root module.

Code: Imports across the root package tree are rewritten from /v3 to /v4 for those libraries (client, datasources, datastore, hooks, tests, etc.). testservice is updated the same way and temporarily pins integration modules (Consul, DynamoDB, Redis) at pseudo-versions so contract tests can build against this branch.

Docs: The README section describing the launchdarkly_easyjson build tag is removed; config.go / package_info.go comments now reference go-sdk-common/v4 paths.

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