[Admin] `licenses/` directory out of date

[WardBrian]

Description

The licenses/ directory includes a file called eigen-license.txt which contains the LGPL3 license. Eigen's license has changed to be "primarily" MPL2, see: https://gitlab.com/libeigen/eigen/-/blob/3.4/COPYING.README

The file dependencies.txt in that same folder is also considerably out of date, both for things like supported compilers/versions and it seems to be pre-stanc3 and has some notes on building the now-deleted stanc2 compiler.