Upgrade the used JDK in the project to v21 (#3582)

* Upgrade the used JDK in the project to v21

* Use it for CI too

* Centralise java language version

* Fix deprecations, tests and lint issues

* Fix coverage taking into account `@Preview` annotated code.

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Co-authored-by: Benoit Marty <benoit@matrix.org>

libraries/core/src/main/kotlin/io/element/android/libraries/core/uri/UrlUtils.kt

@@ -7,11 +7,11 @@
 
 package io.element.android.libraries.core.uri
 
-import java.net.URL
+import java.net.URI
 
 fun String.isValidUrl(): Boolean {
     return try {
-        URL(this)
+        URI(this).toURL()
         true
     } catch (t: Throwable) {
         false