[ci] Split JDK matrix pipelines per OS (#15534) (#15537)

This commit splits the generic Buildkite pipelines introduced
in #15520 for JDK tests to separate pipelines for Linux and Windows.

(cherry picked from commit 07147b3e40)

Co-authored-by: Dimitrios Liappis <dimitrios.liappis@gmail.com>

.buildkite/jdk_matrix_pipeline.yml

@@ -1,3 +0,0 @@
-steps:
-  - label: "Test JDK matrix pipeline"
-    command: "echo 'Hello world'"

.buildkite/linux_jdk_matrix_pipeline.yml

@@ -0,0 +1,3 @@
+steps:
+  - label: "Test Linux JDK matrix pipeline"
+    command: "echo 'Hello world'"

.buildkite/windows_jdk_matrix_pipeline.yml

@@ -0,0 +1,3 @@
+steps:
+  - label: "Test Windows JDK matrix pipeline"
+    command: "echo 'Hello world'"