[MINOR][DOCS][MESOS] Clarify that Mesos version is a lower bound.

## What changes were proposed in this pull request?

Clarify that 0.21 is only a **minimum** requirement.

## How was the this patch tested?

It's a doc change, so no tests.

Author: Iulian Dragos <jaguarul@gmail.com>

Closes #11271 from dragos/patch-1.