Change gitCommitIdLength lower bound to 7

## Issue

**version**: 1.9.0

**usage context**: _any_

**Problem description**:

Docs for settings say:
```xml
integer
```

In Github all short commit IDs usually have ```7``` digits length, like in this project: [c14645a](https://github.com/jgitver/jgitver-maven-plugin/commit/c14645a1900b47ac92c516319a987e6173e93006).
If the Git commit ID from _jgitver_ is used to create docker containers, for example, it is useful to have their tag correspond to a commit ID on Github. This is not possible, if the lower bound for the ```gitCommitIdLength``` is 8 instead of 7.