Replace gradle-git with grgit

[viktorkovarik-devcors]

The gradle-git is deprecated and it's replaced by grgit for a while:
https://github.com/tschulte/gradle-semantic-release-plugin/blob/master/build.gradle#L43
can you please update it? Removal of old subdependency from maven/gradle repositories caused the breakage of the plugin.

* What went wrong:
A problem occurred configuring root project 'spring-boot-starters'.
> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find org.ajoberstar:grgit:1.9.3.
     Searched in the following locations:
       - file:/Users/viktorkovarik/.m2/repository/org/ajoberstar/grgit/1.9.3/grgit-1.9.3.pom
       - https://repo.maven.apache.org/maven2/org/ajoberstar/grgit/1.9.3/grgit-1.9.3.pom
       - https://maven.pkg.github.com/goeuro/java-packages/org/ajoberstar/grgit/1.9.3/grgit-1.9.3.pom
       - https://plugins.gradle.org/m2/org/ajoberstar/grgit/1.9.3/grgit-1.9.3.pom
     Required by:
         project : > de.gliderpilot.semantic-release:de.gliderpilot.semantic-release.gradle.plugin:1.4.2 > gradle.plugin.de.gliderpilot.gradle.semantic-release:gradle-semantic-release-plugin:1.4.2
         project : > de.gliderpilot.semantic-release:de.gliderpilot.semantic-release.gradle.plugin:1.4.2 > gradle.plugin.de.gliderpilot.gradle.semantic-release:gradle-semantic-release-plugin:1.4.2 > org.ajoberstar:gradle-git:1.7.2