Remove unnecessary backport config since we don't backport TO master

Author: mark-vieira

.backportrc.json

@@ -1,15 +1,13 @@
 {
   "upstream": "elastic/elasticsearch",
   "targetBranchChoices": [
-    { "name": "master", "checked": true },
     "7.16",
     "7.15",
     "7.14",
     "6.8"
   ],
   "targetPRLabels": ["backport"],
   "branchLabelMapping": {
-    "^v8.0.0$": "master",
     "^v(\\d+).(\\d+).\\d+$": "$1.$2"
   }
 }

build.gradle

@@ -103,8 +103,7 @@ tasks.register("verifyVersions") {
 
     // Make sure backport bot config file is up to date
     JsonNode backportConfig = new ObjectMapper().readTree(file(".backportrc.json"))
-    List<BwcVersions.UnreleasedVersionInfo> unreleased = BuildParams.bwcVersions.unreleased.collect { BuildParams.bwcVersions.unreleasedInfo(it) }
-    unreleased.each { unreleasedVersion ->
+    BuildParams.bwcVersions.forPreviousUnreleased { unreleasedVersion ->
       boolean valid = backportConfig.get("targetBranchChoices").elements().any { branchChoice ->
         if (branchChoice.isObject()) {
           return branchChoice.get("name").textValue() == unreleasedVersion.branch