Undirected graph support for MG graphs

[jnke2016]

This PR allows our MG implementation to support undirected graph by symmetrizing the dataframe. This will be consistent with the SG implementation.

It also

• replaces the deprecated cudf call append by concat
• update Neighborhood sampling's docstring and code to only support int32 vertices and labels by Dylan

closes #2240
closes #2174
closes #2264 by raising a ValueError and updating the docstring

[codecov-commenter]

Codecov Report

Merging #2247 (9b9eba1) into branch-22.06 (38be932) will decrease coverage by 0.98%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-22.06    #2247      +/-   ##
================================================
- Coverage         70.82%   69.83%   -0.99%     
================================================
  Files               170      173       +3     
  Lines             11036    11435     +399     
================================================
+ Hits               7816     7986     +170     
- Misses             3220     3449     +229     

Impacted Files	Coverage Δ
python/cugraph/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/centrality/__init__.py	100.00% <ø> (ø)
...graph/cugraph/centrality/betweenness_centrality.py	89.65% <ø> (ø)
...thon/cugraph/cugraph/centrality/katz_centrality.py	88.23% <ø> (-1.24%)	⬇️
python/cugraph/cugraph/community/egonet.py	97.36% <ø> (ø)
...ython/cugraph/cugraph/community/ktruss_subgraph.py	85.29% <ø> (ø)
...cugraph/cugraph/dask/centrality/katz_centrality.py	25.00% <ø> (ø)
python/cugraph/cugraph/dask/common/input_utils.py	22.13% <ø> (+0.57%)	⬆️
python/cugraph/cugraph/dask/common/mg_utils.py	30.43% <ø> (-7.07%)	⬇️
python/cugraph/cugraph/dask/common/part_utils.py	19.23% <ø> (-0.94%)	⬇️
... and 104 more

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[ajschmidt8]

Removing ops-codeowners from the required reviews since it doesn't seem there are any file changes that we're responsible for. Feel free to add us back if necessary.

[rlratzel]

@gpucibot merge