sysutils/snmp_exporter: update to 0.28.0

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changes since the previous port version (0.20.0) are too numerous to
list here but can be found at the GitHub releases page:

https://github.com/prometheus/snmp_exporter/releases

this release introduces a breaking change in the configuration file
format. to address this:

• add a new pkg-message that explains the situation on upgrade.

• add an UPDATING entry to warn ports users about the change.

• add a new port called 'sysutils/snmp_exporter20' containing the old 0.20.0 version; users who don't want to immediately update their configuration can instead switch to this new port to continue using the old version.

PR: 282759
Approved by: fabian.freyer@physik.tu-berlin.de (maintainer timeout)