refactor: use StandardCharsets.UTF_8 constant

[elharo]

@silvolu

[codecov]

Codecov Report

Merging #532 (6d807e9) into master (ecfc6a2) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #532      +/-   ##
============================================
- Coverage     80.06%   80.05%   -0.02%     
  Complexity      421      421              
============================================
  Files            28       28              
  Lines          1966     1965       -1     
  Branches        209      209              
============================================
- Hits           1574     1573       -1     
  Misses          283      283              
  Partials        109      109              

Impacted Files	Coverage Δ	Complexity Δ
..._http/java/com/google/auth/oauth2/OAuth2Utils.java	66.66% <ø> (-0.55%)	12.00 <0.00> (ø)
...java/com/google/auth/oauth2/GoogleCredentials.java	61.36% <100.00%> (ø)	10.00 <0.00> (ø)
.../google/auth/oauth2/ServiceAccountCredentials.java	82.44% <100.00%> (ø)	49.00 <0.00> (ø)
...uth/oauth2/ServiceAccountJwtAccessCredentials.java	75.64% <100.00%> (ø)	40.00 <0.00> (ø)
...p/java/com/google/auth/oauth2/UserCredentials.java	80.28% <100.00%> (ø)	32.00 <0.00> (ø)

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