[MINDEXER-130] produce sha256 hashes for index files

**[Jeff Hodges](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jmhodges)** opened **[MINDEXER-130](https://issues.apache.org/jira/browse/MINDEXER-130?redirect=false)** and commented

(If this is the wrong project, sorry.)

It'd be nice if the index files produced and published included SHA256 hashes on top of the currently available SHA1 files.

I'm looking at https://repo1.maven.org/maven2/.index/ and seeing only SHA1 files produced, for instance.

SHA1 is no longer secure and various interfaces that give you SHA1 files (like Guava's Hashing API) will toss up deprecation notices.

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No further details from [MINDEXER-130](https://issues.apache.org/jira/browse/MINDEXER-130?redirect=false)

Start by locating the code that produces and publishes the index files, then trace how the existing SHA1 files are generated. The change is complete when published index files also have corresponding SHA256 hashes, alongside the current SHA1 files.

Written by the indexing model from the issue text.