ConcurrentModificationException onUpdatePressed(LibraryManagerUI)

Hi! I'd like to report an issue (bug) with the Arduino IDE.
macOS 10.15.5
Arduino IDE 1.8.13 (Official build)

The issue can be reproduced with the following scenario.
On a freshly started Arduino IDE, I check for updatable packages, by opening Tools > Board: "..." > Boards Manager...
I wait to load, then I select Type Updatable. After this I click Close.
Then I open the Library Manager by clicking Sketch > Include Library > Manage Libraries...
While it loads, the below exception is thrown.

```
java.util.ConcurrentModificationException
java.lang.RuntimeException: java.util.ConcurrentModificationException
at cc.arduino.contributions.libraries.ui.LibraryManagerUI.lambda$onUpdatePressed$2(LibraryManagerUI.java:207)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.util.ConcurrentModificationException
at java.util.ArrayList.sort(ArrayList.java:1464)
at java.util.Collections.sort(Collections.java:177)
at cc.arduino.contributions.libraries.ui.LibrariesIndexTableModel.updateContributions(LibrariesIndexTableModel.java:208)
at cc.arduino.contributions.libraries.ui.LibrariesIndexTableModel.update(LibrariesIndexTableModel.java:138)
at cc.arduino.contributions.libraries.ui.LibraryManagerUI.lambda$onUpdatePressed$2(LibraryManagerUI.java:204)
... 1 more
```

Reproduce the macOS Arduino IDE 1.8.13 sequence: check for updatable board packages, close Boards Manager, then open Library Manager while it loads. Start with LibraryManagerUI.java around lines 204-207 and LibrariesIndexTableModel.java around lines 138 and 208, then verify the sequence no longer throws ConcurrentModificationException.

Written by the indexing model from the issue text.