Java: prepare release java-0.2.8-alpha (#2426)

### Motivation and Context

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### Description

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### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Co-authored-by: GitHub <[email protected]>

java/samples/pom.xml

@@ -4,7 +4,7 @@
 
     <groupId>com.microsoft.semantic-kernel</groupId>
     <artifactId>samples</artifactId>
-    <version>0.2.8-alpha-SNAPSHOT</version>
+    <version>0.2.8-alpha</version>
     <packaging>pom</packaging>
     <name>Semantic Kernel Samples Parent</name>
     <description>Parent pom for sample code for using the Semantic Kernel project</description>

java/samples/sample-code/pom.xml

@@ -4,7 +4,7 @@
     <parent>
         <groupId>com.microsoft.semantic-kernel</groupId>
         <artifactId>samples</artifactId>
-        <version>0.2.8-alpha-SNAPSHOT</version>
+        <version>0.2.8-alpha</version>
         <relativePath>../pom.xml</relativePath>
     </parent>
 

java/samples/semantickernel-samples-guice/pom.xml

@@ -6,7 +6,7 @@
     <parent>
         <groupId>com.microsoft.semantic-kernel</groupId>
         <artifactId>samples</artifactId>
-        <version>0.2.8-alpha-SNAPSHOT</version>
+        <version>0.2.8-alpha</version>
         <relativePath>../pom.xml</relativePath>
     </parent>