Upgrade firebase-admin, avoid gRPC version conflict

The latest firebase-admin (v6.12.2) depends on grpc-api v1.24.1,
which is as close as we can get to the grpc v1.25.0 specified in
build.gradle.  Currently, the firebase-admin version requirement
is causing the build to download grpc v1.10.1 jars, but at runtime,
grpc 1.25.0 jars are loaded.

Transitive dependencies grpc-netty-shaded, grpc-protobuf,
grpc-protobuf-lite, grpc-protobuf-stub, grpc-protobuf-grpclb
(v1.23.0) are also excluded.

Eliminating version conflicts saves a dev time thinking about which
source jar to choose from while examining exception stack traces.

Partial fix for bisq-network#4086

build.gradle

@@ -34,7 +34,7 @@ configure(subprojects) {
         easybindVersion = '1.0.3'
         easyVersion = '4.0.1'
         findbugsVersion = '3.0.2'
-        firebaseVersion = '6.2.0'
+        firebaseVersion = '6.12.2'
         fontawesomefxVersion = '8.0.0'
         fontawesomefxCommonsVersion = '9.1.2'
         fontawesomefxMaterialdesignfontVersion = '2.0.26-9.1.2'
@@ -428,7 +428,16 @@ configure(project(':relay')) {
         compile project(':common')
         compile "com.sparkjava:spark-core:$sparkVersion"
         compile "com.turo:pushy:$pushyVersion"
-        compile "com.google.firebase:firebase-admin:$firebaseVersion"
+        implementation("com.google.firebase:firebase-admin:$firebaseVersion") {
+            exclude(module: 'grpc-core')
+            exclude(module: 'grpc-api')
+            exclude(module: 'grpc-context')
+            exclude(module: 'grpc-protobuf')
+            exclude(module: 'grpc-protobuf-lite')
+            exclude(module: 'grpc-protobuf-stub')
+            exclude(module: 'grpc-protobuf-grpclb')
+            exclude(module: 'grpc-netty-shaded')
+        }
         compile "commons-codec:commons-codec:$codecVersion"
     }
 }