Metrics

Fix database count for latest incremental backup metrics #133:
- Reuse the database count from the selected differential backup when it also represents the latest incremental backup;
- Wait for all parallel database count queries to finish before completing metric collection;
- Add regression tests for full, differential and incremental backup selection.

Other

Run the systemd service as the postgres user and group by default #130. Set User= and Group= in the service file if pgBackRest runs under a different OS user.
Bump default pgBackRest version to v2.60.0 #128, #131, #132.
Bump Go version and dependencies #129:
- Bump Go version to 1.27.0;
- Bump github.com/prometheus/client_golang to v1.24.1;
- Bump github.com/prometheus/common to v0.71.0;
- Bump github.com/prometheus/exporter-toolkit to v0.19.0;
- Update other dependencies and the exporter-toolkit documentation link.
Update build tooling #129, #131:
- Bump golangci-lint to v2.14.0;
- Bump GoReleaser to v2.18.0.