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`GITEA_TRACE`, `GITEA_RUNNER_CAPACITY`, `GITEA_RUNNER_FILE`, `GITEA_RUNNER_ENVIRON`, `GITEA_RUNNER_ENV_FILE` env vars (superseded by the config file)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/pkg/report/reporter.go

@@ -639,7 +639,7 @@ func (r *Reporter) handleCommand(originalContent, command, value string) *string
 }
 
 func (r *Reporter) parseLogRow(entry *log.Entry) *runnerv1.LogRow {
-	content := strings.TrimRightFunc(entry.Message, func(r rune) bool { return r == '\r' || r == '\n' })
+	content := strings.TrimRight(entry.Message, "\r\n")
 
 	matches := cmdRegex.FindStringSubmatch(content)
 	if matches != nil {