refactor: using slices.Contains to simplify the code (#9918)

### Purpose

This is a [new function](https://pkg.go.dev/slices@go1.21.0#Contains)
added in the go1.21 standard library, which can make the code more
concise and easy to read.

### Testing

Describe what testing has been done, and how the reviewer can test the
change
if new tests are not included.

### Screenshots

If this is a GUI change, include screenshots of the change. If not,
please
feel free to just delete this section.

### Documentation

If this is a user visible change (including API and protocol changes),
add a link here
to the corresponding pull request on https://github.com/syncthing/docs
or describe
the documentation changes necessary.

## Authorship

Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.

Signed-off-by: dashangcun <907225865@qq.com>
Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/config/optionsconfiguration.go

@@ -9,6 +9,7 @@ package config
 import (
 	"fmt"
 	"runtime"
+	"slices"
 
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/rand"
@@ -268,13 +269,7 @@ func (opts OptionsConfiguration) AutoUpgradeEnabled() bool {
 }
 
 func (opts OptionsConfiguration) FeatureFlag(name string) bool {
-	for _, flag := range opts.FeatureFlags {
-		if flag == name {
-			return true
-		}
-	}
-
-	return false
+	return slices.Contains(opts.FeatureFlags, name)
 }
 
 // LowestConnectionLimit is the lower of ConnectionLimitEnough or

lib/model/devicedownloadstate.go

@@ -7,6 +7,8 @@
 package model
 
 import (
+	"slices"
+
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sync"
 )
@@ -40,12 +42,7 @@ func (p *deviceFolderDownloadState) Has(file string, version protocol.Vector, in
 		return false
 	}
 
-	for _, existingIndex := range local.blockIndexes {
-		if existingIndex == index {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(local.blockIndexes, index)
 }
 
 // Update updates internal state of what has been downloaded into the temporary