Replace custom Base64 with java.util.Base64

[exceptionfactory]

This pull request replaces the custom net.schmizz.sshj.common.Base64 class with the standard java.util.Base64 implementation.

Following the changes in #874 to make Java 8 the minimum required version, maintaining a standalone Base64 implementation is no longer required.

Although the Base64 class has remained stable, it does not have standalone unit tests. Switching to java.util.Base64 provides the same functionality and all tests pass using the standard implementation.

[sonatype-lift]

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[codecov-commenter]

Codecov Report

Merging #879 (8ff2de2) into master (39a7be9) will increase coverage by 2.72%.
The diff coverage is 93.54%.

@@             Coverage Diff              @@
##             master     #879      +/-   ##
============================================
+ Coverage     65.34%   68.06%   +2.72%     
+ Complexity     1441     1399      -42     
============================================
  Files           209      208       -1     
  Lines          8060     7463     -597     
  Branches        746      637     -109     
============================================
- Hits           5267     5080     -187     
+ Misses         2396     2038     -358     
+ Partials        397      345      -52     

Impacted Files	Coverage Δ
...hj/transport/verification/FingerprintVerifier.java	82.75% <85.71%> (+10.03%)	⬆️
...sshj/transport/verification/KnownHostMatchers.java	91.48% <100.00%> (ø)
.../sshj/userauth/keyprovider/OpenSSHKeyFileUtil.java	83.33% <100.00%> (ø)
...sshj/userauth/keyprovider/OpenSSHKeyV1KeyFile.java	84.66% <100.00%> (ø)
...nomus/sshj/userauth/keyprovider/bcrypt/BCrypt.java	96.46% <100.00%> (+13.64%)	⬆️
...sshj/transport/verification/OpenSSHKnownHosts.java	57.14% <100.00%> (-0.22%)	⬇️
...chmizz/sshj/userauth/keyprovider/PuTTYKeyFile.java	80.90% <100.00%> (ø)

... and 3 files with indirect coverage changes

[hierynomus]

Thanks for another pull request! Looking good and merged.