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`GITEA_TRACE`, `GITEA_RUNNER_CAPACITY`, `GITEA_RUNNER_FILE`, `GITEA_RUNNER_ENVIRON`, `GITEA_RUNNER_ENV_FILE` env vars (superseded by the config file)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/client/header.go

@@ -6,6 +6,4 @@ package client
 const (
 	UUIDHeader  = "x-runner-uuid"
 	TokenHeader = "x-runner-token"
-	// Deprecated: could be removed after Gitea 1.20 released
-	VersionHeader = "x-runner-version"
 )