Use sha256 checksums to verify maven dependencies

[sstone]

Using sha256 is as secure, diffs will be easier, sha256 checksums are more commonly used and it makes more sense for a bitcoin project.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.07%. Comparing base (12df4ce) to head (ddb1bb7).
Report is 8 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2998      +/-   ##
==========================================
+ Coverage   86.02%   86.07%   +0.05%     
==========================================
  Files         227      228       +1     
  Lines       20371    20452      +81     
  Branches      815      846      +31     
==========================================
+ Hits        17524    17605      +81     
  Misses       2847     2847              

see 11 files with indirect coverage changes

[pm47]

I was able to reproduce an identical checksum file by following this procedure:

1. delete ~/.m2/wrapper
2. delete ~/.sbt
3. delete .mvn/checksums/checksums-central.sha256
4. run ./mvnw clean install scoverage:report -DskipTests -Daether.artifactResolver.postProcessor.trustedChecksums.record

I think this should be documented in BUILD.md

[sstone]

I was able to reproduce an identical checksum file by following this procedure:

1. delete `~/.m2/wrapper`

2. delete `~/.sbt`

3. delete `.mvn/checksums/checksums-central.sha256`

4. run `./mvnw clean install scoverage:report -DskipTests -Daether.artifactResolver.postProcessor.trustedChecksums.record`

I think this should be documented in BUILD.md

Done in 83c5280