Remove stale Gitea 1.20 compatibility shims (#978)

The runner already enforces Gitea v1.21+ (see the `connect.CodeUnimplemented` check in `daemon.go`), so several shims kept for v1.20 compatibility have been dead since 2023:

- `compatibleWithOldEnvs` — the `GITEA_DEBUG`, `GITEA_TRACE`, `GITEA_RUNNER_CAPACITY`, `GITEA_RUNNER_FILE`, `GITEA_RUNNER_ENVIRON`, `GITEA_RUNNER_ENV_FILE` env vars (superseded by the config file)
- `VersionHeader` (`x-runner-version`) and the `version` param of `client.New`
- `AgentLabels` field in `RegisterRequest` (replaced by `Labels`)

Also replaces a verbose `strings.TrimRightFunc` closure with `strings.TrimRight(s, "\r\n")` in the log row parser.

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Nicolas <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/978
Reviewed-by: Nicolas <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-committed-by: silverwind <[email protected]>

internal/app/cmd/daemon.go

@@ -132,7 +132,6 @@ func runDaemon(ctx context.Context, daemArgs *daemonArgs, configFile *string) fu
 			cfg.Runner.Insecure,
 			reg.UUID,
 			reg.Token,
-			ver.Version(),
 		)
 
 		runner := run.NewRunner(cfg, reg, cli)