Docs: Replace deprecated 'compile' with 'implementation' in Gradle snippet

[yadavay-amzn]

The Gradle snippet on the Releases page uses the compile configuration, which was removed in Gradle 7. Updated to implementation to match current Gradle conventions.

Change:
site/docs/releases.md line 48: compile → implementation

Testing:

1. Verified mkdocs build runs successfully from site/ directory (the monorepo plugin warning about nightly docs is pre-existing and unrelated)

2. Ran pymarkdownlnt scan site/docs/releases.md — no lint issues on the changed line

3. Confirmed Iceberg's own build.gradle uses implementation (68 instances) and zero bare compile configurations:

   $ grep -c "implementation " build.gradle
   68
   $ grep -c "^[[:space:]]*compile " build.gradle
   0
   

4. Searched all docs for any other Gradle compile references — none found:

   $ grep -rn "compile '" site/ docs/ --include="*.md"
   (no results)
   

5. Verified the rendered markdown context is correct — the Gradle snippet now reads:

   dependencies {
     implementation 'org.apache.iceberg:iceberg-core:{{ icebergVersion }}'
   }
   

Closes #15811