Further align synthesized annotation `toString()` with modern JDKs

[sbrannen]

While working on #35938, I noticed that the toString() implementation for annotations on modern JDKs no longer includes value= for an annotation with a single value attribute.

I then noticed that the toString() implementation on modern JDKs also differs in the following ways.

• d is no longer appended to double values (since it's superfluous).
• There is no longer a space after a (byte) cast.
• Hexidecimal values in bytes are formatted using lowercase instead of uppercase.

Since these changes are simple to make in our toString() implementation for synthesized annotations, we should update SynthesizedMergedAnnotationInvocationHandler accordingly.