Unify both of Sebastiáns in mailmap

[yarikoptic]

With such a chance would look like

❯ git shortlog -sn
   276  Sebastián Galkin
    66  Matthew Iannucci
    23  Orestis Herodotou
    18  Deepak Cherian
    14  Ryan Abernathey
    12  Joe Hamman
     7  dependabot[bot]
     3  Joseph Hamman
     2  Aimee Barciauskas
     2  Tom Nicholas

whenever without:

❯ git shortlog -sn
   166  Sebastian Galkin
   110  Sebastián Galkin
    66  Matthew Iannucci
    23  Orestis Herodotou
    18  Deepak Cherian
    14  Ryan Abernathey
    12  Joe Hamman
     7  dependabot[bot]
     3  Joseph Hamman
     2  Aimee Barciauskas
     2  Tom Nicholas

[paraseba]

Thank you @yarikoptic ! Didn't know this trick!