Conditionally build BWC projects in parallel (#39396)

This commit sets the BWC projects to build in parallel if Gradle was
invoked with parallal project execution enabled. This substantially
speeds up the time of building the BWC projects since there are many
dependent projects needed to build a BWC version.

distribution/bwc/build.gradle

@@ -207,6 +207,9 @@ bwcVersions.forPreviousUnreleased { VersionCollection.UnreleasedVersionInfo unre
             } else if (showStacktraceName.equals("ALWAYS_FULL")) {
                 args "--full-stacktrace"
             }
+            if (gradle.getStartParameter().isParallelProjectExecutionEnabled()) {
+                args "--parallel"
+            }
             standardOutput = new IndentingOutputStream(System.out, bwcVersion)
             errorOutput = new IndentingOutputStream(System.err, bwcVersion)
             configure extraConfig