fix!: privatize deprecated constructor

[elharo]

@kolea2 one of last two deprecated methods to clean up before releasing 1.0

see #238

Can we take care of this now? I think https://www.github.com/googleapis/google-auth-library-java/issues/350 means we can do this now.

[elharo]

[ERROR] 7009: com.google.auth.oauth2.ServiceAccountJwtAccessCredentials: Accessibility of method 'public ServiceAccountJwtAccessCredentials(java.lang.String, java.lang.String, java.security.PrivateKey, java.lang.String)' has been decreased from public to private

is expected and this library is pre-1.0

[codecov]

Codecov Report

Merging #473 (40af346) into master (269b49d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #473   +/-   ##
=========================================
  Coverage     79.98%   79.98%           
  Complexity      420      420           
=========================================
  Files            28       28           
  Lines          1964     1964           
  Branches        208      208           
=========================================
  Hits           1571     1571           
  Misses          284      284           
  Partials        109      109           

Impacted Files	Coverage Δ	Complexity Δ
...uth/oauth2/ServiceAccountJwtAccessCredentials.java	75.64% <ø> (ø)	40.00 <0.00> (ø)

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