docd: add slog and JSON logging

[jonathaningram]

Remove -log-level flag from docd in favor of callers passing the X-Debug: true header. Example CURL:

curl -F "input=@test.pdf" "http://127.0.0.1:8888/convert?readability=1" -H "X-Debug: true" -H "X-Cloud-Trace-Context: 1234/5678"

Outputs in logs:

{"time":"2023-10-30T05:47:29.208212088Z","severity":"DEBUG","logging.googleapis.com/sourceLocation":{"function":"main.(*convertServer).convert","file":"/app/docd/convert.go","line":31},"message":"Readability is on","logging.googleapis.com/trace":"projects/xxxx/traces/1234","logging.googleapis.com/spanId":"5678"}
{"time":"2023-10-30T05:47:29.208321755Z","severity":"INFO","logging.googleapis.com/sourceLocation":{"function":"main.(*convertServer).convert","file":"/app/docd/convert.go","line":75},"message":"Received file","filename":"test-good.pdf","mimeType":"application/pdf","logging.googleapis.com/trace":"projects/xxxx/traces/1234","logging.googleapis.com/spanId":"5678"}

Change minimum Go version to 1.21.