Naming of build files for LHC

[freddieknets]

The make_sequence files (in the sequences folder) for the LHC are now named as lhcRunX_thin.madx

I propose to not name it in function of the Run, but in function of the year, because I noticed that for Run 2 there are different sequence files per year (the differences are not very large, but sometimes won't work with optics from another year).

So I propose to change the name of the current file lhcRun3_thin.madx into lhc2021_thin.madx (to allow for different sequence files to develop in the years to come after, and for consistency with Run 2 in which we need to organise the files base on year).

[rdemaria]

good idea for me