Remove and gitignore pubspec.lock files

[Widcket]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

The auth0_flutter package includes /pubspec.lock in its .gitignore. However, the .gitignore of the auth0_flutter_platform_interface package does not.
Yet both pubspec.lock files are present in the repository.

This causes the following warning when publishing auth0_flutter:

Given that:

• Pub docs do not recommend committing lock files (for libraries)
• We added them for CI convenience reasons that no longer apply after the move to GitHub actions

This PR removes both pubspec.lock files, and adds /pubspec.lock to the .gitignore of auth0_flutter_platform_interface package. And also adds **/doc/api to it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (253209a) 96.08% compared to head (f2030d9) 90.31%.
Report is 23 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #340      +/-   ##
============================================
- Coverage     96.08%   90.31%   -5.78%     
============================================
  Files            97       49      -48     
  Lines          1611      475    -1136     
  Branches        331        0     -331     
============================================
- Hits           1548      429    -1119     
+ Misses           49       46       -3     
+ Partials         14        0      -14     

Flag	Coverage Δ
auth0_flutter	100.00% <100.00%> (ø)
auth0_flutter_android	?
auth0_flutter_ios	?
auth0_flutter_platform_interface	87.03% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...lib/src/web/extensions/user_profile_extension.dart	100.00% <100.00%> (ø)
...lutter_platform_interface/lib/src/credentials.dart	100.00% <100.00%> (ø)

... and 50 files with indirect coverage changes

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