Add @finnsky as core maintainer for Umami

Problem/Motivation

@finnsky has been instrumental in several major improvements to the umami profile. Most of the components that have been converted to SDC have been completed by him almost single-handedly.

#3390334: Create new Badge component for recipe
#3378588: Convert Umami's node.html.twig to use SDC components
#3365497: Create new SDC component for Umami Banner
#3390344: Create new List component for Umami
#3397532: Fix Umami container max-width when toolbar or navigation expanded.

It would be great to have someone with so much knowledge of how SDC works in Umami as a maintainer of Umami before SDC becomes stable in Drupal core.

I've approached him and asked if he is interested in being a co-maintainer, he has said yes.

Proposed resolution

1. @finnsky to confirm publicly and confirm that:
   • He has read the Drupal core governance, including especially How decisions are made.
   • He understands the high-level description of maintainers and is comfortable with it.
   • He is committed to providing constructive feedback as defined in the issue etiquette documentation.
2. patch/MR to maintainers.txt
3. review
4. rtbc
5. commit
6. update maintainers on node/3060

Remaining tasks

As above