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

[isaacs]

.github/workflows/build.yml:548:

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. #22790 (merged) set the >= 2.8.3 floor, and #23256 (merged) bumped the setup-deno action. Neither touched the dead matrix.deno-version expression.

[linear-code]

JS-3524