Fix clazy issue "analitzaplot/private/functiongraphfactory.cpp:152:25:
warning: allocating an unneeded temporary container [clazy-container-anti-pattern]".

It complains about the lack of precision on the float there whereas
elsewhere it doesn't. Adding the highp still works on other devices so
we have all bases covered.

BUG: 460239

In KF we already looking for 5.15.2
and looking something lower than that
is a bit meaningless

So hopefully mistakes like the one fixed by the previous commit will be
noticed earlier.

2: FAIL!  : AnalitzaTest::testCorrection(lambda1) Compared values are not the same
2:    Actual   (evalResult.toString()): "a<2"
2:    Expected (result)               : "false"

Introduced with 093061c0.

In [1] the build system was changed to use Eigen3's imported targets
provided upstream. These are only available with the cmake config
files, not the FindEigen3 module (also upstream). With the basic
signature of find_package() it searches in Module mode first, finding
Eigen3 via FindEigen3.cmake and erroring out afterwards because
Eigen3::Eigen is an unknown target.
Forcing find_package to use the Config mode fixes that.

[1] 7f3d443c

...by using QT_MAJOR_VERSION instead of QT_REQUIRED_VERSION/hard-coding
"5", introduced by 05dc6051.

CCMAIL: montel@kde.org