JDK 27: Adapt to `sourcePositions.getStartPosition` changes in b16

[wmdietl]

b16 fails with

/home/runner/work/checker-framework/checker-framework/framework/src/main/java/org/checkerframework/framework/source/SourceChecker.java:1911: warning: [removal] getStartPosition(CompilationUnitTree,Tree) in SourcePositions has been deprecated and marked for removal
        long start = sourcePositions.getStartPosition(currentRoot, tree);
> Task :framework:compileJava
                                    ^
/home/runner/work/checker-framework/checker-framework/framework/src/main/java/org/checkerframework/framework/source/SourceChecker.java:1912: warning: [removal] getEndPosition(CompilationUnitTree,Tree) in SourcePositions has been deprecated and marked for removal
        long end = sourcePositions.getEndPosition(currentRoot, tree);
                                  ^
error: warnings found and -Werror specified
/home/runner/work/checker-framework/checker-framework/framework/src/main/java/org/checkerframework/common/basetype/BaseTypeVisitor.java:519: warning: [removal] getStartPosition(CompilationUnitTree,Tree) in SourcePositions has been deprecated and marked for removal
                            positions.getStartPosition(root, expected),
                                     ^

Which comes from a recent change in the JDK. I'll suppress these warnings and file a follow-up issue.

Originally posted by @wmdietl in #1617 (comment)