build: downgrade spotless to version 6.13.0

Later versions of spotless do not support Java 8 anymore. Refer to:
diffplug/spotless#1337 (comment)

.github/workflows/continuous-deployment.yml

@@ -26,7 +26,7 @@ jobs:
           uses: actions/setup-java@v3
           with:
             distribution: 'adopt'
-            java-version: 11
+            java-version: 8
         # Install Node in the provided runner
         - name: Install Node
           uses: actions/setup-node@v3

.github/workflows/continuous-testing.yml

@@ -10,7 +10,7 @@ jobs:
       strategy:
         matrix:
           os: [windows, macos, ubuntu]
-          java-version: [11, 17]
+          java-version: [8, 11, 17]
       runs-on: ${{matrix.os}}-latest
       steps:
         # Install the specified version of Java in the provided runner

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 scala-version = "3.3.0"                             # required to run scala
 scalatest-version = "3.2.15"                        # required to test scala
 scalatestplusjunit-version = "3.2.16.0"             # required to run scalatest directly within IntelliJ
-spotless-version = "6.19.0"                         # required to apply different style checkers within the project
+spotless-version = "6.13.0"                         # required to apply different style checkers within the project
 scalafmt-version = "3.7.1"                          # style checker for scala
 wartremover-version = "0.1.1"                       # code linter for scala
 git-semantic-versioning-version = "1.1.9"           # assigns a project version depending on the existing git tags