refactor: rename `FlightSQL` constant to `SQL`

[bednar]

Closes #70

Proposed Changes

Rename FlightSQL constant to SQL.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] Tests pass
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

[codecov[bot]]

Codecov Report

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

Project coverage is 83.09%. Comparing base (f0bf1b6) to head (5fe866c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ======================================= Coverage 83.09% 83.09% ======================================= Files 12 12 Lines 994 994 ======================================= Hits 826 826 Misses 138 138 Partials 30 30 ```

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[jstirnaman]

Thanks!