Use HTTPS instead of SSH for Git submodules

Some of the Git submodules previously used SSH instead of HTTPS, which:
- Causes errors in environments where SSH auth isn't configured
  (such as when people prefer to use HTTPS auth for GitHub, but
   use a different SSH key for other use-cases, and use a wildcard
   in the SSH config).
- Is unnecessary, given these are public repos so don't need
  auth to read.

This fixes the following errors I get locally:

```
$ git submodule update
Cloning into '/Users/emorley/src/buildpacks-jvm/buildpacks/maven/test-apps/heroku-java-getting-started'...
[email protected]: Permission denied (publickey).
fatal: Could not read from remote repository.
```

GUS-W-13745392.

.gitmodules

@@ -4,7 +4,8 @@
 
 [submodule "buildpacks/sbt/test-apps/heroku-scala-getting-started"]
 	path = buildpacks/sbt/test-apps/heroku-scala-getting-started
-	url = [email protected]:heroku/scala-getting-started.git
+	url = https://github.com/heroku/scala-getting-started.git
+
 [submodule "buildpacks/maven/test-apps/heroku-java-getting-started"]
 	path = buildpacks/maven/test-apps/heroku-java-getting-started
-	url = git@github.com:heroku/java-getting-started.git
+	url = https://github.com/heroku/java-getting-started.git