Rename `random_lcg` to more explanatory name

[rzehumat]

Description

This is related to #2486 .

The files containing Random number generator are

• include/random_lcg.h,
• src/random_lcg.cpp.

However, as PCG is now used, the name random_lcg looks a bit misleading. I was wondering if other name -- maybe random or random_pcg would be better.

Alternatives

The easiest alternative to renaming would be to keep it as it is, since renaming the code, which is used quite often, might cause merge conflicts to pull requests opened before such change.

Compatibility

Renaming might result in merge conflicts. so it might be dangerous. On the other hand, other name can be more explanatory.