Linux compatibility issues and update build settings

.gitattributes

@@ -0,0 +1,15 @@
+# Line endings are not cosmetic for these.
+#
+# The Maven wrapper is a shell script executed on Linux runners: committed with CRLF, its shebang
+# reads as an interpreter named "/bin/sh\r", which does not exist, and the build dies with exit 127
+# before Maven ever starts. Checking these out as LF everywhere is what keeps that from recurring.
+mvnw            text eol=lf
+*.sh            text eol=lf
+Dockerfile      text eol=lf
+*.yml           text eol=lf
+*.yaml          text eol=lf
+
+# Windows counterparts, which need the opposite.
+mvnw.cmd        text eol=crlf
+*.bat           text eol=crlf
+*.ps1           text eol=crlf

Library-Management-System-Version-2/pom.xml

@@ -260,8 +260,12 @@
 				<artifactId>maven-compiler-plugin</artifactId>
 				<version>3.11.0</version>
 				<configuration>
-					<source>${java.version}</source>
-					<target>${java.version}</target>
+					<!--
+						release, not source/target: those emit 21 bytecode but still compile against
+						whatever JDK is installed, so an API added after 21 builds on a developer's
+						machine and fails on a JDK 21 runner. release pins the API surface too.
+					-->
+					<release>${java.version}</release>
 					<annotationProcessorPaths>
 						<path>
 							<groupId>org.projectlombok</groupId>