CI: remove io engine parametrization, always run with tokio-epoll-uring

Running both combinations bloats CI times too much.

So, we switch to tokio-epoll-uring for 2-3 days, so that the code gets exposure.
Then we switch it back a few days before the next release so that the
`std-fs` engine gets coverage, because that's what we're going to
continue using in prod for the time being.

.github/workflows/build_and_test.yml

@@ -432,7 +432,6 @@ jobs:
       matrix:
         build_type: [ debug, release ]
         pg_version: [ v14, v15, v16 ]
-        pageserver_virtual_file_io_engine: [ std-fs, tokio-epoll-uring ]
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -455,7 +454,7 @@ jobs:
           TEST_RESULT_CONNSTR: ${{ secrets.REGRESS_TEST_RESULT_CONNSTR_NEW }}
           CHECK_ONDISK_DATA_COMPATIBILITY: nonempty
           BUILD_TAG: ${{ needs.tag.outputs.build-tag }}
-          PAGESERVER_VIRTUAL_FILE_IO_ENGINE: ${{ matrix.pageserver_virtual_file_io_engine }}
+          PAGESERVER_VIRTUAL_FILE_IO_ENGINE: tokio-epoll-uring
 
       - name: Merge and upload coverage data
         if: matrix.build_type == 'debug' && matrix.pg_version == 'v14'
@@ -475,7 +474,6 @@ jobs:
         # the amount of groups (N) should be reflected in `extra_params: --splits N ...`
         pytest_split_group: [ 1, 2, 3, 4 ]
         build_type: [ release ]
-        pageserver_virtual_file_io_engine: [ std-fs, tokio-epoll-uring ]
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -492,7 +490,7 @@ jobs:
           VIP_VAP_ACCESS_TOKEN: "${{ secrets.VIP_VAP_ACCESS_TOKEN }}"
           PERF_TEST_RESULT_CONNSTR: "${{ secrets.PERF_TEST_RESULT_CONNSTR }}"
           TEST_RESULT_CONNSTR: "${{ secrets.REGRESS_TEST_RESULT_CONNSTR_NEW }}"
-          PAGESERVER_VIRTUAL_FILE_IO_ENGINE: "${{ matrix.pageserver_virtual_file_io_engine }}"
+          PAGESERVER_VIRTUAL_FILE_IO_ENGINE: tokio-epoll-uring
       # XXX: no coverage data handling here, since benchmarks are run on release builds,
       # while coverage is currently collected for the debug ones