Fix #750 — startup failure on macOS / Windows with JRE 8
Releases v8.25 and v8.27 were built with maven.compiler.target=11, so their JARs shipped with class file major version 55 and refused to launch under any JRE 8 — macOS users saw the generic "Java Application launch failed" dialog and Windows users hit UnsupportedClassVersionError. v8.23 still worked because its release was cut before that pom change and shipped class file major 52.
v8.28 reverts source/target to 1.8 and rewrites the single Java 11-only call site (Files.readString in FileMergerDialog) using Files.readAllBytes + new String(..., UTF_8). Java 8 compatibility is kept for now. Verified: the JAR has class file major 52 on every project class and starts cleanly under Temurin 11.