feat!: regenerate from protocol v237.1.3 and move the module to /v6

[mogita]

Problem

getstream-go is generated from protocol openapi-v236.2.0. The current release, openapi-v237.1.3, changes the shape of six generated types, which breaks Go call sites even though nothing was removed from the API.

Solution

Regenerated against openapi-v237.1.3 and moved the module to /v6.

ChannelMemberRequest.UserID   string                -> *string
ChannelMemberRequest.User     *UserResponse         -> *MemberUserRequest   (new type)
Reminder{Created,Updated,Deleted,Notification}Event.Reminder
                              *ReminderResponseData -> ReminderResponseData

No endpoint, type or field was removed, and no field was added. oasdiff between the two specs reports 44 errors, all request-property-type-changed, matching the first two rows.

Go resolves a v2+ module only if go.mod carries the matching suffix, and release-please never rewrites it, so go.mod and all 76 self-imports move to /v6 here. This has to be on main before the major Release PR merges, or the guard in release.yml refuses to tag. CHANGELOG.md and MIGRATION_v4_to_v5.md keep their /v5 references as historical record.

MIGRATION_v5_to_v6.md documents the four call-site changes. The 15 affected lines in the hand-written tests use the existing PtrTo helper.

How to verify

1. head -1 go.mod shows module github.com/GetStream/getstream-go/v6.
2. grep -rl "getstream-go/v5" . --exclude-dir=.git lists only CHANGELOG.md and MIGRATION_v4_to_v5.md.
3. ./generate.sh produces no diff beyond .spec-version if a newer protocol release has landed since.
4. go build ./... && go vet ./... && go test -short ./... all pass.

Review instructions

Merge order matters. PR #145 is open proposing 5.4.0 from base 5.3.0. If this merges first, release-please recomputes that same PR as 6.0.0 and 5.4.0 never ships. Merge #145 first if you want 5.4.0 to exist.

Expect the Release workflow to go red on the push that lands this PR: main will carry go.mod at /v6 while the manifest still says 5.3.0 or 5.4.0, and the guard fails on that mismatch by design. It blocks only the tagging step, not the Release PR, and it clears once the 6.0.0 Release PR merges.