Collapse SYNC+ASYNC per-version workflows into one matrix

Wave D item 18, minimal-scope variant. The five workflow files that
run the identical SYNC+ASYNC suite against different HDF5 versions
(1.14.0, 1.14.1, 2.0.0, 2.1.1, 2.2.0) collapse into a single matrix
workflow. The four older SYNC-only files (1.10.4, 1.10.7, 1.10.8,
1.12.0) stay as-is because their test set is a strict subset and
they don't run the ASYNC leg; collapsing them would require
conditional gates that make the matrix noisier than the win.

## New file

.github/workflows/h5bench-hdf5-matrix.yml
  - strategy.fail-fast: false so one bad version does not cancel
    the others
  - strategy.matrix.hdf5_version: [1.14.0, 1.14.1, 2.0.0, 2.1.1, 2.2.0]
  - container.image: hpcio/hdf5-${{ matrix.hdf5_version }}
  - job name: "h5bench (HDF5 ${{ matrix.hdf5_version }})" so each
    matrix leg reports as e.g. "h5bench (HDF5 2.0.0)", matching the
    old per-version workflow's job name as closely as possible.

## Deleted

.github/workflows/h5bench-hdf5-1.14.0.yml
.github/workflows/h5bench-hdf5-1.14.1.yml
.github/workflows/h5bench-hdf5-2.0.0.yml
.github/workflows/h5bench-hdf5-2.1.1.yml

## Branch protection

Required-check names WILL shift. The old checks appeared as
"h5bench (HDF5 X.X.X) / h5bench"; the new ones appear as
"h5bench / h5bench (HDF5 X.X.X)". Any required-check rules on
develop/master that name the old paths must be updated before
this merges, or the required checks will hang.

## Coordination with #166 (2.2.0)

If #166 lands first, its h5bench-hdf5-2.2.0.yml must be deleted on
rebase of this PR (it is redundant with the matrix leg). If this PR
lands first, #166 should drop the per-version file and just push the
image.

Author: jeanbez