fix: Keycloak url base

[PrincipeAgustin]

Description
OIDC integration with Keycloak does not work unless Keycloak is configured with /auth in the context path.

Fix Keycloak url conforming to: alerta/alerta#1683
Fixes #570

Changes

• Remove /auth from Keycloak url.

Checklist

• Pull request is limited to a single purpose
• Code style/formatting is consistent
• All existing tests are passing

Collaboration
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pull request's compare branch. If the pull request author wants greater
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(that is, anyone with push access to the upstream repository) permission
to commit to the pull request's compare branch

See https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork

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