Remove redundant now grgit Gradle plugin

The `grgit` Gradle plugin has been used to determine modified files to update their Copyright.
After migrating to a `-present` pattern we don't need to track modified files anymore.
Therefore, we don't need to manage `org.ajoberstar.grgit` plugin, too.

**Cherry-pick to `6.5.x` & `6.4.x`**

Author: artembilan

.github/dependabot.yml

@@ -21,7 +21,6 @@ updates:
           - org.gradle.*
           - com.github.spotbugs
           - io.spring.*
-          - org.ajoberstar.grgit
           - org.antora
           - com.google.protobuf
           - io.micrometer:micrometer-docs-generator
@@ -67,7 +66,6 @@ updates:
           - org.gradle.*
           - com.github.spotbugs
           - io.spring.*
-          - org.ajoberstar.grgit
           - org.antora
           - com.google.protobuf
           - io.micrometer:micrometer-docs-generator
@@ -113,7 +111,6 @@ updates:
           - org.gradle.*
           - com.github.spotbugs
           - io.spring.*
-          - org.ajoberstar.grgit
           - org.antora
           - com.google.protobuf
           - io.micrometer:micrometer-docs-generator

build.gradle

@@ -17,7 +17,6 @@ buildscript {
 plugins {
     id 'base'
     id 'io.spring.nohttp' version '0.0.11' apply false
-    id 'org.ajoberstar.grgit' version '5.3.2'
     id 'io.spring.dependency-management' version '1.1.7'
     id 'org.jetbrains.dokka' version '1.9.20'
     id 'org.antora' version '1.0.0'
@@ -47,14 +46,6 @@ ext {
     linkScmConnection = 'scm:git:git://github.com/spring-projects/spring-integration.git'
     linkScmDevConnection = 'scm:git:ssh://[email protected]:spring-projects/spring-integration.git'
 
-    modifiedFiles =
-            files()
-                    .from {
-                        files(grgit.status().unstaged.modified)
-                                .filter { f -> f.name.endsWith('.java') || f.name.endsWith('.kt') || f.name.endsWith('.groovy') }
-                    }
-    modifiedFiles.finalizeValueOnRead()
-
     apacheSshdVersion = '2.15.0'
     artemisVersion = '2.41.0'
     aspectjVersion = '1.9.24'