Run NullAway in JSpecify mode

Outside it NullAway takes Guava's @ParametricNullness for @nullable:
PeekingIterator.peek() and next(), Iterables.getLast and
getOnlyElement, Iterators.getLast. Every token taken from a
PeekingIterator<Token> was reported as possibly null. JSpecify mode
reads Guava's type parameters instead, and of NullAway's 102 findings
60 remain.

On javac 21 the mode needs -XDaddTypeAnnotationsToSymbol=true to see
type-use annotations, @nullable among them, on classes read from jars.

Author: abashev

buildSrc/src/main/groovy/open-java-format.error-prone-conventions.gradle

@@ -12,9 +12,12 @@ dependencies {
 
 nullaway {
     annotatedPackages.add('com.palantir')
+    jspecifyMode = true
 }
 
 tasks.withType(JavaCompile).configureEach {
+    // JSpecify mode reads @Nullable on types from class files, which javac 21 does only with this flag.
+    options.compilerArgs.add('-XDaddTypeAnnotationsToSymbol=true')
     options.errorprone {
         // Annotation processor output (Immutables) is not ours to fix.
         excludedPaths = '.*/build/generated/.*'