Update dependency popper.js to v2

| datasource | package                     | from   | to     |
| ---------- | --------------------------- | ------ | ------ |
| cdnjs      | popper.js/umd/popper.min.js | 1.16.1 | 2.11.8 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

bootstrap/index.html

@@ -536,7 +536,7 @@ <h4 class="card-title">Dark card title</h4>
 
 <!-- jQuery first, then Popper.js, then Bootstrap JS -->
 <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
-<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script>
+<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/2.11.8/umd/popper.min.js" integrity="sha512-TPh2Oxlg1zp+kz3nFA0C5vVC6leG/6mm1z9+mA81MI5eaUVqasPLO8Cuk4gMF4gUfP5etR73rgU/8PNMsSesoQ==" crossorigin="anonymous"></script>
 <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
 
 </body>