[dart][dart-dio] Bump Dio to 5.0.0

[kuhnroyal]

This PR updates the dart-dio generator to use dio: ^5.0.0.

Closes #14281

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CC @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[ahmednfwela]

LGTM, there is just a small typo

[asaarnak]

diox got ownership of dio github and migrated to dio: 5.0.0

[ahmednfwela]

that's great news, then we can just replace all Diox strings in this PR to just Dio5 and change the dio_import back to normal dio

[kuhnroyal]

that's great news, then we can just replace all Diox strings in this PR to just Dio5 and change the dio_import back to normal dio

We depend on lomsa-dev/http-mock-adapter#140 since we have tests using this package.

[AlexV525]

Now dio 5.0 will not imply the default content type, please ensure it's valid in requests.

[kuhnroyal]

We have always manually set the content-type if it is specified in the spec. See https://github.com/OpenAPITools/openapi-generator/blob/master/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/api/fake_api.dart#L147

[kuhnroyal]

I think thats enough for now.

[kuhnroyal]

Tests are not running on min SDK due to https://github.com/lomsa-dev/http-mock-adapter/pull/140/files#r1104810809

[kuhnroyal]

This is ready I think, the Git test dependency can be switched to a released version later, it is pinned to a commit.