Upgrade agent-mtd-identifiers

[platops-pr-bot]

• Scala can be updated from 2.12.15 to 2.13.8 in build.sbt - See confluence https://confluence.tools.tax.service.gov.uk/x/L4HJHw
• uk.gov.hmrc.bootstrap-backend-play-28 in project/AppDependencies.scala can be removed - its already provided by uk.gov.hmrc.agent-kenshoo-monitoring
• System property lookup can be removed in conf/application-json-logger.xml - Bootstrap's LoggerModule allows configuration of loggers without this - See confluence https://confluence.tools.tax.service.gov.uk/x/9oH1FQ
• Add uk.gov.hmrc.bootstrap-test-play-28 in project/AppDependencies.scala to provide recommended test dependencies. Use a variable to keep version in sync with bootstrap-play