Edge case generation and shrinking improvements

New and Enhanced Features

• Shrinking behaviour of arbitraries created with
  Combinators.withBuilder(..) is now much better, especially much faster.

• Upgrade to JUnit Platform 1.7.0

Breaking Changes

• A maximum of 1000 (instead of 10000) edge cases is generated per arbitrary.

• Arbitraries that allow nullables through Arbitrary.injectNull() or
  annotation @WithNull will now shrink their values to null if possible.

Bug Fixes

• With a lot of edge cases sometimes only edge cases were generated.
  Now the minimum ratio is 1 edge case in 3 generating steps.

• Warning about combinatorial explosion of edge cases generation is
  now logged only once.

• Edge cases for oneOf(..) and frequencyOf(..) generators are now correctly being shrunk.