ci: remove code.json, update scripts, fix release workflow

[wpbonelli]

Fixes for the release automation after discussion with @jdhughes-usgs

• don't version code.json, just generate when needed (and update update_version.py)
• add retries for metadata file gen in release.yml (USGS endpoints are unreliable)
• fix autocommit/pr contents: version.txt, pymake/config.py, and README.md

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.051%. Comparing base (006b6ba) to head (ced4a26).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #190 +/- ## ============================================= - Coverage 77.946% 70.051% -7.895% ============================================= Files 20 20 Lines 3496 3496 ============================================= - Hits 2725 2449 -276 - Misses 771 1047 +276 ``` [see 11 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/pymake/pull/190/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)