proposal: x/oauth2: deprecate provider specific packages

[seankhliao]

Proposal Details

Proposal

Deprecate the following provider specific subpackages of golang.org/x/oauth2.
Format importers (via pkg.go.dev), package name.

• 55 amazon
• 197 bitbucket
• 6 cern
• 331 facebook
• 15 fitbit
• 31 foursquare
• 813 github
• 34 gitlab
• 14 heroku
• 6 hipchat
• 33 instagram
• 1 jira
• 44 kakao
• 61 linkedin
• 7 mailchimp
• 12 mailru
• 6 mediamath
• 147 microsoft
• 3 nokiahealth
• 8 odnoklassniki
• 8 paypal
• 53 slack
• 79 spotify
• 2 stackoverflow
• 105 twitch
• 6 uber
• 51 vk
• 8 yahoo
• 56 yandex

Motivation

Since 2018, golang/oauth2#325 / CL 147459
we've stopped adding new provider specific packages, instead adding new endpoints to just the
endpoint package.

Some of them are for entities that no longer exist (e.g. nokiahealth),
others refer to versioned api endpoints that are no longer available
(e.g. facebook v3.2 ended on may 4, 2021).

I think sufficient time has passed that we can deprecate the provider specific packages.

I left out the google tree, as it's much more extensive in its apis.
I believe there's work to move to cloud.google.com/go/auth for those
cc @hickford