Error caused by indexes of array from getMessages()

Problem/Motivation

When deleting the first message of a thread, the second message will not be shown in the thread.

This problem comes after this commit:
https://git.drupalcode.org/project/private_message/-/commit/4d3384197ab0...

Before, the array returned by getMessages() was always indexed starting on zero and incremented by one. After this commit, the array of messages can be indexed in different ways and when the first message is deleted it will start in 1.

This has an impact in filterUserDeletedMessages() because it relies on the old indexes to slice the array.

Even inside the method getMessages(), it seems that there is a related problem because it is also supposing that the index is always zero for the banned filter.

Steps to reproduce

# Create two messages in a thread;
# Delete the first message;
# Check that both messages disappear from the thread.

Proposed resolution

The array of messages inside getMessages() should be as before, with incremental indexes starting on zero.