Upgrade to Spring Boot 3.1.0-RC2 (#3917)

#### What type of PR is this?

/kind cleanup
/area core

#### What this PR does / why we need it:

Upgrade to Spring Boot 3.1.0-RC2.

See https://github.com/spring-projects/spring-boot/releases/tag/v3.1.0-RC2 and https://github.com/spring-projects/spring-boot/releases/tag/v3.1.0-RC1 for more.

#### Which issue(s) this PR fixes:

Fixes https://github.com/halo-dev/halo/issues/3816

#### Does this PR introduce a user-facing change?

```release-note
升级 Spring Boot 至 3.1.0-RC2
```

api/build.gradle

@@ -9,6 +9,7 @@ description = 'API of halo project, connecting by other projects.'
 
 repositories {
     mavenCentral()
+    maven { url 'https://repo.spring.io/milestone' }
 }
 
 dependencies {

application/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'org.springframework.boot' version '3.0.6'
+    id 'org.springframework.boot' version '3.1.0-RC2'
     id 'io.spring.dependency-management' version '1.1.0'
     id "com.gorylenko.gradle-git-properties" version "2.3.2"
     id "checkstyle"
@@ -22,7 +22,6 @@ repositories {
     mavenCentral()
 
     maven { url 'https://repo.spring.io/milestone' }
-    mavenLocal()
 }
 
 

platform/application/build.gradle

@@ -1,7 +1,7 @@
 import org.springframework.boot.gradle.plugin.SpringBootPlugin
 
 plugins {
-    id 'org.springframework.boot' version '3.0.6' apply false
+    id 'org.springframework.boot' version '3.1.0-RC2' apply false
     id 'java-platform'
     id 'halo.publish'
     id 'signing'