feat: remove absence alerts. make common alert file

[keithf4]

Description

Remove unneeded absence alerts. Make a common alerts file and move the ExporterDown alert there.

Please indicate what kind of change your PR includes (multiple selections are acceptable):

• Bugfix
• Enhancement
• Breaking Change
• Documentation

PRs should be against existing issues, so please list each issue using a separate 'closes' line:

closes #461

If this PR depends on another PR or resolution of another issue, please indicate that here using a separate 'depends' line for each dependency.

depends on #

If you have an external dependency (packages, portal updates, etc), add the 'BLOCKED' tag to your PR.

Testing

None of the testing listed below is optional.

• Installation method:
  • Binary install from source, version:
  • OS package repository, distro, and version:
  • Local package server, version:
  • Custom-built package, version:
  • Other:
• PostgreSQL, Specify version(s):
• docs tested with hugo version(s):

Code testing

Have you tested your changes against:

• RedHat/CentOS
• Ubuntu
• Not applicable

If your code touches sql_exporter, have you:

• Tested against all versions of PostgreSQL affected
• Ensure that exporter runs with no scrape errors
• Not applicable

If your code touches node_exporter, have you:

• Ensure that exporter runs with no scrape errors
• Not applicable

If your code touches Prometheus, have you:

• Ensured all configuration changes pass promtool check config
• Ensured all alert rule changes pass promtool check rules
• Prometheus runs without issue
• Alertmanager runs without issue
• Not applicable

If your code touches Grafana, have you:

• Ensured Grafana runs without issue
• Ensured relevant dashboards load without issue
• Not applicable

Checklist:

• I have made corresponding changes to:
  • the documentation
  • the release notes
  • the upgrade doc