The `immediateFlush` option in both rolling file appenders is ignored

## Description

Both the `RollingFileAppender` and `RollingRandomAccessFileAppender` plugins offer a boolean flag, `immediateFlush`, to allow for immediate flushing of buffers when writing data. This option is ultimately discarded when creating the corresponding `RollingFileManager` and `RollingRandomAccessFileManager` instances.

## Configuration

**Version:** 2.x and main

**Operating system:** all

**JDK:** all

## Logs

n/a

## Reproduction

The amusing part is that we have tons of tests that set `immediateFlush` to `true` or `false`, yet none of these tests verify that it does anything.