Add JitPack repo and update libsuperuser dependency (#2001)

## Description
Back in 2021 [JFrog
announced](https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/)
that they are deprecating the JCenter repository. It's still read-only
but it shouldn't be depended upon.

I was trying to remove it in the other PR:
https://github.com/syncthing/syncthing-android/pull/2000, but then I
found out a single dependency still needs it:
[libsuperuser](https://github.com/Chainfire/libsuperuser).
The most "up to date" version [can be found in
JitPack](https://jitpack.io/#Chainfire/libsuperuser).
I had to update the version manually as dependabot would never pick it
up until JitPack is added to the list of repositories.

## Changes
* add the JitPack as a new repository to the project
* update the `libsuperuser` dependency to version `1.1.1`

app/build.gradle

@@ -5,7 +5,7 @@ plugins {
 }
 
 dependencies {
-    implementation 'eu.chainfire:libsuperuser:1.1.0.202004101746'
+    implementation 'eu.chainfire:libsuperuser:1.1.1'
     implementation 'com.google.android.material:material:1.8.0'
     implementation 'com.google.code.gson:gson:2.10.1'
     implementation 'org.mindrot:jbcrypt:0.4'

build.gradle

@@ -23,6 +23,7 @@ allprojects {
     repositories {
         google()
         jcenter()
+        maven { url = "https://jitpack.io" }
     }
 }