Generate compiled SpEL expressions using Java 17 byte code level

[sbrannen]

When generating dynamic classes with CGLIB or compiling SpEL expressions, we currently generate byte code with Java 8 byte code level.

For Spring Framework 7, we should generate byte code with Java 17 byte code level to align with the Java 17 baseline introduced in Spring Framework 6.0.

[sbrannen]

Unfortunately, it is not possible to raise the byte code level beyond Java 1.8 for classes generated using CGLIB due to the fact that CGLIB generates STATICHOOK methods which set static final fields outside the initializer method <clinit> (i.e., a static initialization block).

Attempting to raise the level to Java 17 (or even Java 9) results in exceptions like the following.

Caused by: java.lang.IllegalAccessError: Update to static final field org.example.MyBean$$SpringCGLIB$$0.CGLIB$THREAD_CALLBACKS attempted from a different method (CGLIB$STATICHOOK1) than the initializer method <clinit>
	at org.example.MyBean$$SpringCGLIB$$0.CGLIB$STATICHOOK1(<generated>)
	at org.example.MyBean$$SpringCGLIB$$0.<clinit>(<generated>)

We will therefore leave the version set to 1.8 for CGLIB.

• See also: Question: what is the static hook used for? cglib/cglib#95 (comment)