deno: CI runs one Deno version, and a matrix reference is dead

`.github/workflows/build.yml:548`:

```yaml
deno-version: ${{ matrix.deno-version || 'v2.8.3' }}
```

The job has no `strategy: matrix:` block, so `matrix.deno-version` is always empty and the version is always `v2.8.3`. It reads like a matrix that was planned and never wired up.

Related stale note in `dev-packages/deno-integration-tests/suites/orchestrion-mysql/test.ts:19`:

```
// The orchestrion runtime hook only works as a FIRST import inside the entry
// graph in Deno 2.8.0 through 2.8.2.
// TODO: revisit a `--import` or `--preload` approach once Deno 2.8.3 ships.
```

Deno 2.9 is out. The TODO is due.

**Work item.** Either add the matrix the expression expects, or drop the dead `matrix.deno-version ||`. Separately, re-check the `--import` / `--preload` path on 2.9 and update or delete the TODO.
**Prior art ***(related history only)*. No issue. getsentry/sentry-javascript#22790 (merged) set the >= 2.8.3 floor, and getsentry/sentry-javascript#23256 (merged) bumped the `setup-deno` action. Neither touched the dead `matrix.deno-version` expression.

Start with .github/workflows/build.yml:548 and dev-packages/deno-integration-tests/suites/orchestrion-mysql/test.ts:19. Run the relevant Deno integration job on Deno 2.9 and check the --import/--preload behavior. Done means the workflow no longer has a dead matrix reference and the stale TODO is updated or removed, with the integration tests passing.

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