Build: Use branch specific refspec sysprop for bwc builds (#29299)

This commit changes the sysprop for overriding the branch bwc builds use
to be branch specific. There are 3 different bwc branches built, but all
of them currently read the exact same sysprop. For example, with this change
and current branches, you can now specify eg `-Dtests.bwc.refspec.6.x=my_6x`
and it will build only next-minor-snapshot with that branch, while
next-bugfix-snapshot will continue to use 5.6.

TESTING.asciidoc

@@ -476,12 +476,12 @@ branch. Finally, on a release branch, it will test against the most recent relea
 === BWC Testing against a specific remote/branch
 
 Sometimes a backward compatibility change spans two versions. A common case is a new functionality
-that needs a BWC bridge in and an unreleased versioned of a release branch (for example, 5.x).
+that needs a BWC bridge in an unreleased versioned of a release branch (for example, 5.x).
 To test the changes, you can instruct Gradle to build the BWC version from a another remote/branch combination instead of
-pulling the release branch from GitHub. You do so using the `tests.bwc.remote` and `tests.bwc.refspec` system properties:
+pulling the release branch from GitHub. You do so using the `tests.bwc.remote` and `tests.bwc.refspec.BRANCH` system properties:
 
 -------------------------------------------------
-./gradlew check -Dtests.bwc.remote=${remote} -Dtests.bwc.refspec=index_req_bwc_5.x
+./gradlew check -Dtests.bwc.remote=${remote} -Dtests.bwc.refspec.5.x=index_req_bwc_5.x
 -------------------------------------------------
 
 The branch needs to be available on the remote that the BWC makes of the
@@ -496,7 +496,7 @@ will need to:
 will contain your change.
 . Create a branch called `index_req_bwc_5.x` off `5.x`. This will contain your bwc layer.
 . Push both branches to your remote repository.
-. Run the tests with `./gradlew check -Dtests.bwc.remote=${remote} -Dtests.bwc.refspec=index_req_bwc_5.x`.
+. Run the tests with `./gradlew check -Dtests.bwc.remote=${remote} -Dtests.bwc.refspec.5.x=index_req_bwc_5.x`.
 
 == Test coverage analysis
 

distribution/bwc/build.gradle

@@ -91,7 +91,7 @@ subprojects {
 
   String buildMetadataKey = "bwc_refspec_${project.path.substring(1)}"
   task checkoutBwcBranch(type: LoggedExec) {
-    String refspec = System.getProperty("tests.bwc.refspec", buildMetadata.get(buildMetadataKey, "${remote}/${bwcBranch}"))
+    String refspec = System.getProperty("tests.bwc.refspec.${bwcBranch}", buildMetadata.get(buildMetadataKey, "${remote}/${bwcBranch}"))
     dependsOn fetchLatest
     workingDir = checkoutDir
     commandLine = ['git', 'checkout', refspec]