ci: bump actions/github-script from v6 to v7 in local workflows

## Problem

Two local workflows still pin `actions/github-script@v6`, which runs on Node 16. GitHub has deprecated the Node 16 Actions runtime, and every run of these workflows currently produces a deprecation warning.

Affected call sites (6 total):

- `.github/workflows/new-spec-release.yml` — 5 uses (steps at lines 35, 56, 63, 93, 128)
- `.github/workflows/update-spec.yaml` — 1 use (step at line 46)

`actions/github-script@v7` runs on Node 20 and is otherwise a drop-in upgrade for these scripts, which only use stable `fs` APIs plus `github-token` passthrough.

## Proposed solution

Bump `actions/github-script@v6` to `@v7` in both local workflow files. No script logic changes required.

## Context

This is a follow-up to #1194 / #1195 (checkout v3 → v4). Together these finish moving the local workflows off the Node 16 Actions runtime. Centrally-managed workflows (those carrying the `# This action is centrally managed in https://github.com/asyncapi/.github/` header) are out of scope and should be updated upstream.

I'd like to submit the PR.

Start by opening .github/workflows/new-spec-release.yml and .github/workflows/update-spec.yaml, then inspect the six listed actions/github-script call sites. Update each local workflow reference from v6 to v7 without changing script logic, and confirm no v6 references remain in these two files.

Written by the indexing model from the issue text.