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`GITEA_TRACE`, `GITEA_RUNNER_CAPACITY`, `GITEA_RUNNER_FILE`, `GITEA_RUNNER_ENVIRON`, `GITEA_RUNNER_ENV_FILE` env vars (superseded by the config file)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 <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/978
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

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)

internal/app/cmd/register.go

@@ -325,7 +325,6 @@ func doRegister(ctx context.Context, cfg *config.Config, inputs *registerInputs)
 		cfg.Runner.Insecure,
 		"",
 		"",
-		ver.Version(),
 	)
 
 	for {
@@ -366,12 +365,11 @@ func doRegister(ctx context.Context, cfg *config.Config, inputs *registerInputs)
 	}
 	// register new runner.
 	resp, err := cli.Register(ctx, connect.NewRequest(&runnerv1.RegisterRequest{
-		Name:        reg.Name,
-		Token:       reg.Token,
-		Version:     ver.Version(),
-		AgentLabels: ls, // Could be removed after Gitea 1.20
-		Labels:      ls,
-		Ephemeral:   reg.Ephemeral,
+		Name:      reg.Name,
+		Token:     reg.Token,
+		Version:   ver.Version(),
+		Labels:    ls,
+		Ephemeral: reg.Ephemeral,
 	}))
 	if err != nil {
 		log.WithError(err).Error("poller: cannot register new runner")