Update server-id for the migration from maven-deploy-plugin to central-publishing-maven-plugin:

- `maven-deploy-plugin` at least used to look for `sonatype-nexus-snapshots` (probably based on [configuration I removed last week](https://github.com/google/truth/pull/1586/files)).
- `central-publishing-maven-plugin` looks for `central`, even for snapshots.

See google/auto#1965 (comment).

RELNOTES=n/a
PiperOrigin-RevId: 807794922

Author: cpovirk

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
           java-version: 11
           distribution: 'zulu'
           cache: 'maven'
-          server-id: sonatype-nexus-snapshots
+          server-id: central
           server-username: CI_DEPLOY_USERNAME
           server-password: CI_DEPLOY_PASSWORD
       - name: 'Publish'