Migrate melos to v7.x.x

[riojosdev]

Tell us about the task you want to perform and are unable to do so because the feature is not available

While integrating the package openapi_spec version solving conflict error happened due to the not supporting the latest freezed v3.0.6 and freezed_annotation v3.0.0 packages (see issue #815), migrating them to the latest version resulted in another version solving conflict for melos

Because no versions of melos match >6.3.2 <7.0.0 and melos 6.3.2 depends on
  pub_updater ^0.4.0, melos ^6.3.2 requires pub_updater ^0.4.0.
And because every version of apidash_core from path depends on openapi_spec
  from path which depends on pub_updater ^0.5.0, melos ^6.3.2 is incompatible
  with apidash_core from path.
So, because apidash depends on both apidash_core from path and melos ^6.3.2,
  version solving failed.
Failed to update packages.

Describe the solution/feature you'd like us to add

Migrate melos v6.3.2 to the current pre-release 7.0.0-dev.8

Any other feedback you would like to provide regarding the site

Your response here

[animator]

Switch to a lower compatible version of openapi_spec.