refactor: Fix deprecations and other warnings

[mbfakourii]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Solve 25 warnings in dart and flutter package
fix deprecated functions
fix some spelling mistakes
remove unnecessary folder
rename variables names based on dart principles (1)

Closes: #733

Approach

n/a

TODOs before merging

• Add tests
• Add changes to documentation (guides, repository pages, in-code descriptions)
• A changelog entry

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Base: 10.55% // Head: 10.56% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (28bbba8) compared to base (60ca5a4).
Patch coverage: 6.66% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #810   +/-   ##
=======================================
  Coverage   10.55%   10.56%           
=======================================
  Files          47       47           
  Lines        2841     2840    -1     
=======================================
  Hits          300      300           
+ Misses       2541     2540    -1     

Impacted Files	Coverage Δ
...ackages/dart/lib/src/network/parse_live_query.dart	0.00% <0.00%> (ø)
...kages/dart/lib/src/network/parse_websocket_io.dart	0.00% <ø> (ø)
packages/dart/lib/src/utils/parse_live_list.dart	0.00% <0.00%> (ø)
...ackages/flutter/lib/src/utils/parse_live_grid.dart	0.00% <0.00%> (ø)
...ackages/flutter/lib/src/utils/parse_live_list.dart	0.00% <0.00%> (ø)
packages/dart/lib/src/network/parse_query.dart	10.20% <18.51%> (ø)

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[mtrezza]

Since this is not changing the product behavior and doesn't fix a bug, I changed the commit type to refactor.
When this is ready for review please request a review from @parse-community/flutter-sdk-review.

[mbfakourii]

I rechecked the code and it seems to be fine