Migrate to bazel 7

[mlevesquedion]

Bazel 7 (bazelbuild/bazel#18548) seems to introduce some changes that break our tests.

We should look into what is needed to migrate to Bazel 7 and do the migration. For now, we can pin the Bazel version to avoid breakages.

This page has instructions on how to migrate.

I'm not sure exactly what the source of the issue is, but I'm seeing this failure in all our tests (I don't see such failures with Bazel < 7):

exec ${PAGER:-/usr/bin/less} "$0" || exit 1
Executing tests from //stablehlo/testdata:vmap_gather_from_take_indices_name__4__axis_2_enable_xla_True_mode_clip_dynamic.mlir.test
-----------------------------------------------------------------------------
lit.py: $HOME/.local/lib/python3.11/site-packages/lit/TestingConfig.py:151: fatal: unable to parse config file '/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/stablehlo/testdata/vmap_gather_from_take_indices_name__4__axis_2_enable_xla_True_mode_clip_dynamic.mlir.test.runfiles/_main/stablehlo/testdata/lit.site.cfg.py', traceback: Traceback (most recent call last):
  File "$HOME/.local/lib/python3.11/site-packages/lit/TestingConfig.py", line 139, in load_from_path
    exec(compile(data, path, "exec"), cfg_globals, None)
  File "/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/stablehlo/testdata/vmap_gather_from_take_indices_name__4__axis_2_enable_xla_True_mode_clip_dynamic.mlir.test.runfiles/_main/stablehlo/testdata/lit.site.cfg.py", line 21, in <module>
    lit_config.load_config(config, os.path.join(os.environ['TEST_SRCDIR'], 'stablehlo') + "/stablehlo/testdata/lit.cfg.py")
  File "$HOME/.local/lib/python3.11/site-packages/lit/LitConfig.py", line 136, in load_config
    config.load_from_path(path, self)
  File "$HOME/.local/lib/python3.11/site-packages/lit/TestingConfig.py", line 126, in load_from_path
    f = open(path)
        ^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/bazel-working-directory/_main/bazel-out/k8-fastbuild/bin/stablehlo/testdata/vmap_gather_from_take_indices_name__4__axis_2_enable_xla_True_mode_clip_dynamic.mlir.test.runfiles/stablehlo/stablehlo/testdata/lit.cfg.py'

Thanks to @j2kun for filing google/heir#332, which helped me figure out what the issue was and what a fix could look like.