RFR: 8370628: Rename BigInteger::nthRoot to rootn, and similarly for nthRootAndRemainder

[Raffaello Giulietti]

The reason for this last minute renaming is to more closely adhere to the IEEE 754 nomenclature, even in the BigInteger class (and in the internal MutableBigInteger).

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Commit messages:
 - 8370628: Rename BigInteger::nthRoot to rootn, and similarly for nthRootAndRemainder

Changes: https://git.openjdk.org/jdk/pull/27980/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370628
  Stats: 15 lines in 2 files changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/27980.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27980/head:pull/27980

PR: https://git.openjdk.org/jdk/pull/27980