Change encryption key generation algorithm used in the HBase shell

This PR is a follow-up of HBASE-25181 (#2539), where several issues were discussed on the PR:

1. Currently we use PBKDF2WithHmacSHA1 key generation algorithm to generate a secret key for HFile / WalFile encryption, when the user is defining a string encryption key in the hbase shell. This algorithm is not secure enough and not allowed in certain environments (e.g. on FIPS compliant clusters). We are changing it to PBKDF2WithHmacSHA384. It will not break backward-compatibility, as even the tables created by the shell using the new algorithm will be able to load (e.g. during bulkload / replication) the HFiles serialized with the key generated by an old algorithm, as the HFiles themselves already contain the key necessary for their decryption.

Smaller issues to be fixed:

2. Improve the documentation e.g. with the changes introduced by HBASE-25181 and also by some points discussed on the Jira ticket of HBASE-25263.

3. In EncryptionUtil.createEncryptionContext the various encryption config checks should throw IllegalStateExceptions instead of RuntimeExceptions.

4. Test cases in TestEncryptionTest.java should be broken down into smaller tests.

5. TestEncryptionDisabled.java should use ExpectedException JUnit rule to validate exceptions.