source-xero: enable on Cloud and disable OAuth.

[natikgadzhi]

Problem

Souce-Xero limits the number of OAuth users Airbyte's app can have, and we're treading water in securing a partnership.

We already have the selective authenticator in source-xero source code, but we know that on Cloud, we want to restrict new OAuth users.

We do want source-xero to be available on Cloud, though. @nataliekwong mentioned a lot of folks are running it in the Builder as a workaround.

Solution

Remove OAuth authenticator, leave the bearer authenticator only, and release a new version, with a breaking change.

[natikgadzhi]

Unfortunately, Xero has custom components, so it's not as easy as opening it in the builder ;(

[btkcodedev]

Referencing PR: #42128