Add Marek as a project maintainer

Marek is one of the current reviewers. He has consistently
performed all the tasks to earn a top-level maintainer role as
described in the
https://github.com/etcd-io/etcd/blob/main/GOVERNANCE.md#maintainers
Per discussion with existing maintainers, I nominate him for the
maintainer role.
Marek, thank you for your outstanding contributions to the project.

MAINTAINERS

@@ -13,6 +13,7 @@ Gyuho Lee <[email protected]> (@gyuho) pkg:*
 Hitoshi Mitake <[email protected]> (@mitake) pkg:*
 Jingyi Hu <[email protected]> (@jingyih) pkg:*
 Joe Betz <[email protected]> (@jpbetz) pkg:*
+Marek Siarkowicz <[email protected]> (@serathius) pkg:*
 Piotr Tabor <[email protected]> (@ptabor) pkg:*
 Sahdev Zala <[email protected]> (@spzala) pkg:*
 Sam Batschelet <[email protected]> (@hexfusion) pkg:*
@@ -24,6 +25,5 @@ Tobias Grieger <[email protected]> (@tbg) pkg:go.etcd.io/etcd/raft
 
 # REVIEWERS
 Lili Cosic <[email protected]> (lilic@) pkg:*
-Marek Siarkowicz <[email protected]> (serathius@) pkg:*
 Wilson Wang <[email protected]> (wilsonwang371@) pkg:*