Simplify ProtectedIndices

See https://git.eliatra.com/eliatra-suite/eliatra-opensearch-plugin/-/blob/main/security-core/src/main/java/com/eliatra/suite/securityplus/EliatraSuitePlugin.java#L1202

As the internal index name pattern has been simplified to let all internal indices start with .eliatra_internal_ (see https://git.eliatra.com/eliatra-suite/eliatra-opensearch-plugin/-/issues/8) we should simplify the ProtectedIndices functionality accordingly.