Main to 3.0; SF to 6.0; JDK 17

Initial Commit for Spring Framework 6.0/JDK 17

* Fix add-opens for java.util.zip; add distributionSha256Sum to gradle.properties.

Fix remaining errors of migration to SF-6.0

* Remove `lineSeparator` property for `NewlineAtEndOfFile` Checkstyle rule:
Windows checkouts files with a `crlf` line feed
* Add `-Dfile.encoding=UTF-8` jmvarg for Gradle: some source files use non-UTF symbols
* Upgrade to Kotlin `1.5.31`, JSON Path `2.6.0`, Junit `5.8.1`
* Use correct links for external JavaDocs
* Add `Xdoclint:syntax` to JavaDoc Gradle task to suppress JavaDoc warnings for classes
* Use `DuplicatesStrategy.EXCLUDE` for `prepareAsciidocBuild` task
* Align `api` task with SF style

Add `Xdoclint:syntax` to `api` Gradle task

Turns out `api` task generates fresh common JavaDocs,
so it emits the same warnings for missed docs in classes

Remove files restored by rebase

Fix test Java version

Fix build.gradle

.github/workflows/pr-build-workflow.yml

@@ -19,10 +19,10 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - name: Set up JDK 11
+      - name: Set up JDK 17
         uses: actions/setup-java@v1
         with:
-          java-version: 11
+          java-version: 17
 
       - name: Run Gradle
         uses: burrunan/gradle-cache-action@v1