Use the 21.08 branch of rapids-cmake as rmm requires it

[robertmaynard]

Now that rmm uses rapids-cmake we need to update to the 21.08 branch to get the new rapids_cmake_write_version_file function

[codecov-commenter]

Codecov Report

Merging #1683 (2079c87) into branch-21.08 (d01da40) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 2079c87 differs from pull request most recent head f130938. Consider uploading reports for the commit f130938 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.08    #1683      +/-   ##
================================================
+ Coverage         59.77%   59.79%   +0.02%     
================================================
  Files                80       80              
  Lines              3540     3542       +2     
================================================
+ Hits               2116     2118       +2     
  Misses             1424     1424              

Impacted Files	Coverage Δ
python/cugraph/_version.py	44.80% <0.00%> (+0.39%)	⬆️

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[BradReesWork]

@gpucibot merge