You can use a simple Levels adjustment...
Image > Adjustments > Levels
Move the right (white) arrow to the left so it's inside the big histogram indicators, then move the middle (grey) slider to the right to darken the mid-tones a bit. Moving the left arrow (black) to the right will darken the dark tones.
These are merely the basics, play with it until you are happy.
Afterwards, selecting everything except the seal and using Image > Adjustments > Desaturate can remove any slight yellow left behind around the type due to anti-aliasing.
Desaturate removes the hues and makes everything greyscale.