Change new Buffer to Buffer.from after deprecation

[lsarni]

Description

Instead of using new Buffer we should use Buffer.from from here

This code was introduced to fix #6366 so I tested the given user and password after the changes with a swagger: 2.0 file that uses basic auth as explained here:

securityDefinitions:
  basicAuth:
    type: basic

In this case it works as expected, preserving the original bug fix.

While testing I noted that this no longer works for swagger 3.0 where the basic auth is declared as explained here:

securitySchemes:
    basicAuth:     # <-- arbitrary name for the security scheme
      type: http
      scheme: basic

In that case it ends up going into this branch so it doesn't use the custom btoa function.

Motivation and Context

Fixes #6366

How Has This Been Tested?

Tested locally with swagger: "2.0" file that uses basic authentication.

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.