Deprecate alternative names on secrets (#3406)

Closes https://github.com/woodpecker-ci/woodpecker/discussions/2274

# deprecation of alternative names

Instead of
```yaml
secrets:
  - source: some_secret
    target: some_env
```
you now write:
```yaml
environment:
  some_env:
    from_secret: some_secret
```

Also, it's possible to use complex yaml objects in `environment`,
they're turned into json (just like `settings`).

pipeline/frontend/yaml/compiler/compiler.go

@@ -179,10 +179,12 @@ func (c *Compiler) Compile(conf *yaml_types.Workflow) (*backend_types.Config, er
 			cloneSettings["tags"] = "true"
 		}
 		container := &yaml_types.Container{
-			Name:        defaultCloneName,
-			Image:       cloneImage,
-			Settings:    cloneSettings,
-			Environment: c.cloneEnv,
+			Name:     defaultCloneName,
+			Image:    cloneImage,
+			Settings: cloneSettings,
+		}
+		for k, v := range c.cloneEnv {
+			container.Environment[k] = v
 		}
 		step, err := c.createProcess(container, backend_types.StepTypeClone)
 		if err != nil {