refactor!: rename TransactionPreconditions.isValid to `TransactionP…

[overcat]

…reconditions.validate`.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.96%. Comparing base (ec11098) to head (6e26c9c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #666   +/-   ##
=========================================
  Coverage     80.96%   80.96%           
  Complexity     1176     1176           
=========================================
  Files           202      202           
  Lines          4450     4450           
  Branches        374      374           
=========================================
  Hits           3603     3603           
  Misses          616      616           
  Partials        231      231           

Files with missing lines	Coverage Δ
.../main/java/org/stellar/sdk/TransactionBuilder.java	87.71% <100.00%> (ø)
...java/org/stellar/sdk/TransactionPreconditions.java	94.02% <ø> (ø)