Udpate tracker configuration

[josecelano]

Relates to: torrust/torrust-tracker#401

New breaking changes have been applied to the Tracker configuration.

• Renamed log_level to threshold.
• Tracker mode split into listed and private flags.
• Added configuration version.

From:

[logging]
log_level = "info"

[core]
mode = "public"
tracker_usage_statistics = true
inactive_peer_cleanup_interval = 600

To:

version = "2"

[logging]
threshold = "info"

[core]
inactive_peer_cleanup_interval = 600
listed = false
private = false
tracker_usage_statistics = true