deprecate(storage): Mark google::cloud::storage::oauth2 as deprecated

[shubham-up-47]

Marking it deprecated since, the unified credentials documented in @ref guac is recommended now.

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This change is 

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.94%. Comparing base (477ff8a) to head (20846b4). Report is 2 commits behind head on prepare-for-v3.0.0.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15139      +/-   ##
======================================================
- Coverage               92.94%   92.94%   -0.01%     
======================================================
  Files                    2393     2389       -4     
  Lines                  215288   215227      -61     
======================================================
- Hits                   200110   200052      -58     
+ Misses                  15178    15175       -3     

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[ddelgrosso1]

@scotthart is this something we should point at your 'prepare for 3.x' branch?

[shubham-up-47]

Reviewable status: 0 of 9 files reviewed, 1 unresolved discussion

google/cloud/storage/oauth2/anonymous_credentials.h line 40 at r2 (raw file):

 */
class GOOGLE_CLOUD_CPP_DEPRECATED(
    "This class will be removed shortly. "

Everywhere we say removed shortly let's instead say removed in v3.0.0 and later

Code quote:

removed shortly

Done.

[ddelgrosso1]

Repointing this at the prepare-3.0 branch.

[ddelgrosso1]

@shubham-up-47 since these deprecation notices will go into the 3.x release, I think we need to update the messages to say they will be removed in 4.x. We can't remove them in 3.x as customers need a period of deprecation notice.

[shubham-up-47]

@shubham-up-47 since these deprecation notices will go into the 3.x release, I think we need to update the messages to say they will be removed in 4.x. We can't remove them in 3.x as customers need a period of deprecation notice.

Done.