chore: drop changes-branch:next from release workflow

The original release-please cutover (#1361) prescribed `changes-branch:
next` to mirror Stainless's prior topology where SDK codegen landed on
a `next` staging branch. We're retiring that convention in favour of
landing codegen directly on `main` via Increase/stainless's
`tools/promote/` harness — same fix applied to Increase/increase-go
during the v0.591.0 -> v0.592.0 recovery (Increase/increase-go#1279).

With `changes-branch: next` left in place, the workflow runs but
release-please scans `next` (which is exactly at `release: 0.568.0`,
behind `main` by the cutover commit) and finds no commits to release.
Removing the input lets the action use its default (the workflow ref,
i.e. `main`), so subsequent `feat(api)` / `fix(...)` commits open a
release PR as expected.

The `next` branch will be deleted in a follow-up once this lands.

Author: mattt

.github/workflows/release.yml

@@ -20,4 +20,3 @@ jobs:
       - uses: Increase/release-please@v1
         with:
           token: ${{ steps.app-token.outputs.token }}
-          changes-branch: next