FindOpenGL: Drop explicit dependency on X11 (#15268)

In commit 079e8469ab (... OpenGL always needs X11 on Unix, 2002-09-05)
the FindOpenGL module was taught to search for X11 as a dependency of
the OpenGL library.  This was done without a detailed explanation, and
the dependency should not be explicitly needed because OpenGL headers
should not expose applications to X11 APIs directly.

Unfortunately the only way to know if anything legitimately depends on
this behavior (perhaps in static library cases) is to simply remove it
and wait for issues to be reported.  If so, then we can add some kind of
compatibility setting for this later.  Add a release note to draw
attention to this change.

Reported-by: Dainius "GreatEmerald" Masiliūnas <pastas4@gmail.com>
Merge topic 'FindOpenGL-no-X11' into next

d051cbda FindOpenGL: Drop explicit dependency on X11 (#15268)