[repo] Add Martin Costello as a maintainer (#3226)

Author: Kielek

README.md

@@ -84,7 +84,6 @@ repository](https://github.com/open-telemetry/community/blob/main/guides/contrib
 
 [@open-telemetry/dotnet-contrib-approvers](https://github.com/orgs/open-telemetry/teams/dotnet-contrib-approvers):
 
-* [Martin Costello](https://github.com/martincostello), Grafana Labs
 * [Mikel Blanchard](https://github.com/CodeBlanch), Microsoft
 * [Timothy "Mothra" Lee](https://github.com/TimothyMothra)
 
@@ -96,6 +95,7 @@ repository](https://github.com/open-telemetry/community/blob/main/guides/contrib
 [@open-telemetry/dotnet-contrib-maintainers](https://github.com/orgs/open-telemetry/teams/dotnet-contrib-maintainers):
 
 * [Alan West](https://github.com/alanwest), New Relic
+* [Martin Costello](https://github.com/martincostello), Grafana Labs
 * [Piotr Kiełkowicz](https://github.com/Kielek), Splunk
 * [Rajkumar Rangaraj](https://github.com/rajkumar-rangaraj), Microsoft