Specify minimum rust toolchain version

[zsol]

In #613 people reported being unable to compile LibCST on rustc 1.52.1. This PR specifices the minimum version required to compile LibCST.

[codecov-commenter]

Codecov Report

Merging #614 (0180e50) into main (0c509b3) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 0180e50 differs from pull request most recent head af297ac. Consider uploading reports for the commit af297ac to get more accurate results

@@           Coverage Diff           @@
##             main     #614   +/-   ##
=======================================
  Coverage   94.77%   94.78%           
=======================================
  Files         245      245           
  Lines       25140    25238   +98     
=======================================
+ Hits        23826    23921   +95     
- Misses       1314     1317    +3     

Impacted Files	Coverage Δ
libcst/codemod/commands/convert_type_comments.py	95.28% <0.00%> (+0.58%)	⬆️
...demod/commands/tests/test_convert_type_comments.py	96.72% <0.00%> (+1.37%)	⬆️

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