fix: opintopolku-cas-servlet-filter/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-ORGBOUNCYCASTLE-6084022
- https://snyk.io/vuln/SNYK-JAVA-ORGBOUNCYCASTLE-5771339
- https://snyk.io/vuln/SNYK-JAVA-ORGBOUNCYCASTLE-6613080
- https://snyk.io/vuln/SNYK-JAVA-ORGSPRINGFRAMEWORK-7687447

opintopolku-cas-servlet-filter/pom.xml

@@ -34,7 +34,7 @@
     <dependency>
       <groupId>org.springframework.security</groupId>
       <artifactId>spring-security-cas</artifactId>
-      <version>6.1.0</version>
+      <version>6.2.6</version>
     </dependency>
     <dependency>
       <groupId>commons-logging</groupId>