MINOR: [Java] Upgrade bcpkix-jdkon15 dependency to bcpkix-jdkon18 (apache#41458)

### Rationale for this change

Since bcpkix-jdk15on is no longer being maintained and bcpkix-jdkon18 is fully compatible with it, we can replace bcpkix-jdk15on with bcpkix-jdkon18. This will ensure continued support and security for our applications.
FYI: https://www.bouncycastle.org/latest_releases.html

### What changes are included in this PR?

- Upgrade bcpkix-jdkon15 dependency to bcpkix-jdkon18
### Are these changes tested?

- yes

We used the JcaPEMWriter class to convert certificates stored within a KeyStore object into PEM format and subsequently write them to a designated JcaPEMWriter object. Existing test suites provide comprehensive coverage for this functionality.

Authored-by: Calvin Kirs <[email protected]>
Signed-off-by: David Li <[email protected]>

Author: CalvinKirs

java/flight/flight-sql-jdbc-core/pom.xml

@@ -126,8 +126,8 @@
 
     <dependency>
       <groupId>org.bouncycastle</groupId>
-      <artifactId>bcpkix-jdk15on</artifactId>
-      <version>1.70</version>
+      <artifactId>bcpkix-jdk18on</artifactId>
+      <version>1.78.1</version>
     </dependency>
   </dependencies>