Update the Title and Paragraph block default placeholder text colors for standard themes

[derekblank]

When using a standard theme, we are setting a lighter color for the placeholder text color for the Title and the Paragraph blocks. We should update it to the expected color of the recent design changes:

[derekblank]

Hey @geriux 👋 I started working on this one, and just wanted to clarify the intention here. Currently for standard themes, the Post Title and RichText placeholder colors are the same as their globalStyles text color, which makes it seem like there's no distinction between placeholder and "typing" color:

• Post Title code reference
• RichText code reference

For everything else, we fallback to the default RichText placeholder color.

I understand that we'd want the placeholder text color to be lighter than the "typing" text color. But, for standard themes, I'm wondering if we need to retain the the globalStyles text color and just add some opacity (like what was done on the List Item placeholder color), or if we should remove these globalStyles.text placeholder params in those first two code references, so that standard themes fallback to the default RichText placeholder color too. I am leaning toward removing the globalStyles.color params and just having everything have the same placeholder text color, like in this draft PR. WDYT?