ICU-21581 BRSRC 70.1 Version update and regenerate configure for v70.1

ICU-21581 BRSRC 70.1 Version update and regenerate configure for v70.1

In this PR, I am updating the version number from 70.0.1 for the BRS task.
Previously, we had frontloaded part of this, so the diff in this PR are not as numerous.

It has also been decided that we should differentiate frontloaded tasks and RC tasks by having the version numbers being:
Frontload version number: XX.X.X (70.0.1 -> Major.minor.patch)
RC/GA version number: XX.X (70.1 -> Major.minor)

I've added some documentation for this, for future releases :)