Change token logging to trace level (#2247)

[zc-devs]

Changed from info to trace.

Backport of #2247.

Tested on v1.0.2.

Info level:

{"level":"info","time":"2023-08-18T17:10:28Z","message":"no agent config found at '/etc/woodpecker/agent.conf', start with defaults"}
{"level":"error","error":"open /etc/woodpecker/agent.conf: no such file or directory","time":"2023-08-18T17:10:28Z","message":"could not persist agent config at '/etc/woodpecker/agent.conf'"}
{"level":"info","time":"2023-08-18T17:10:28Z","message":"Starting Woodpecker agent with version 'next-d3b96f38' and backend 'kubernetes' using platform 'linux/amd64' running up to 1 pipelines in parallel"}

Trace level:

{"level":"info","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/config.go:41","message":"no agent config found at '/etc/woodpecker/agent.conf', start with defaults"}
{"level":"trace","token":"eyJh...VCJ9.eyJp...1MH0.DHYi...3VIc","time":"2023-08-18T17:06:12Z","caller":"/src/agent/rpc/auth_interceptor.go:96","message":"Token refreshed"}
{"level":"error","error":"open /etc/woodpecker/agent.conf: no such file or directory","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/config.go:70","message":"could not persist agent config at '/etc/woodpecker/agent.conf'"}
{"level":"debug","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/agent.go:200","message":"Agent registered with ID 50"}
{"level":"debug","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/agent.go:223","message":"loaded kubernetes backend engine"}
{"level":"info","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/agent.go:247","message":"Starting Woodpecker agent with version 'next-d3b96f38' and backend 'kubernetes' using platform 'linux/amd64' running up to 1 pipelines in parallel"}
{"level":"debug","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/agent.go:231","message":"created new runner 0"}
{"level":"debug","time":"2023-08-18T17:06:12Z","caller":"/src/cmd/agent/agent.go:238","message":"polling new steps"}
{"level":"debug","time":"2023-08-18T17:06:12Z","caller":"/src/agent/runner.go:54","message":"request next execution"}