Upgrade to Gradle 7.0 (#1712)

* Convert to gradle 7.

* More fixes, regenerated lockfiles.

* Update lockfiles for dependency update.

* Fix show_upgrade_diff for new lockfile format

* Add property for allowInsecureProtocol

Allow us to override the restriction against use of plain HTTP for
communication to dependency repositories.  We need this to be able to use a
local proxy for dependency gathering.

* Checking in missing gradle.lockfile

settings.gradle

@@ -12,12 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Alias this since it collides with the closure variable name.
+def allowInsecure = allowInsecureProtocol
+
 if (!pluginsUrl.isEmpty()) {
   println "Plugins: Using repo $pluginsUrl..."
   pluginManagement {
     repositories {
       maven {
         url pluginsUrl
+        allowInsecureProtocol = allowInsecure == "true"
       }
     }
   }