[REVIEW] Switch language from None to "en" in docs build

[galipremsagar]

Needed for Sphinx 5 compatibility. Should fix the following warning occurring in doc builds.

Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

xref: sphinx-doc/sphinx#10481

[codecov-commenter]

Codecov Report

Merging #939 (0fe9ed2) into branch-22.08 (c89f0db) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           branch-22.08    #939    +/-   ##
=============================================
  Coverage          0.00%   0.00%            
=============================================
  Files                22      16     -6     
  Lines              3073    2092   -981     
=============================================
+ Misses             3073    2092   -981     

Impacted Files	Coverage Δ
dask_cuda/benchmarks/utils.py
dask_cuda/benchmarks/local_cupy_map_overlap.py
dask_cuda/_version.py
dask_cuda/benchmarks/local_cudf_shuffle.py
dask_cuda/benchmarks/local_cudf_merge.py
dask_cuda/benchmarks/local_cupy.py

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c89f0db...0fe9ed2. Read the comment docs.

[jakirkham]

@gpucibot merge