github: Use shared Renovate config

[bkeryan]

What does this Pull Request accomplish?

Use shared Renovate config in https://github.com/ni/python-renovate-config

Switch back to .json file extension. The Renovate docs say

Renovate supports JSONC for .json files and any config files without file extension (e.g. .renovaterc). We also recommend you prefer using JSONC within a .json file to using a .json5 file if you want to add comments.

Why should this Pull Request be merged?

Single-source Renovate config for Python projects.

What testing has been done?

Tested in https://github.com/bkeryan/nitypes-python

[github-actions]

Test Results

   10 files  ±0     10 suites  ±0   21s ⏱️ -2s
  221 tests ±0    221 ✅ ±0  0 💤 ±0  0 ❌ ±0 
2 160 runs  ±0  2 160 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit cb19f09. ± Comparison against base commit b861029.