Swap brightBlack/black in the Solarized color schemes (#6985)

Original notes from @M-Pixel:

> Console applications assume that backgrounds are black, and that
> `lightBlack`/`DarkGrey` are lighter than `black`/`Black`.  This
> assumption is accounted for by all color schemes in `defaults.json`,
> except for the Solarized themes.
> 
> The Solarized Dark theme, in particular, makes `-Parameters` invisible
> against the background in PowerShell, which is obviously an unacceptable
> usability flaw.
> 
> This change makes `black` and `background` to the same (which is common
> throughout the color schemes), and makes `brightBlack` (`DarkGray` in
> .NET) lighter than black (which is obviously more correct given the
> meanings of those words).

Out of the box, we ship a pretty bad behavior.

If I look at all of the existing shipped color schemes--and that
includes things like Tango and One Half--we are universally following a
`background` == `black` rule.

If I consult gnome-terminal or xterm, they do the same thing; Xterm by
default, gnome-terminal for solarized. The background generally matches
color index `0` across all their **dark** schemes. Konsole and
lxterminal disagree and map background to `0 intense` for Solarized.

I want to put our Solarized schemes on a deprecation path, but
unfortunately we still need to ship _something_ for users who we're
going to strand on them.

I'm going to have to swallow my bitter and say that yes, we should
probably just change the index mapping and go with something that works
right out of the box while we figure out how to do perceptual color
nudging and eventually remove bad defaults (like Solarized).

From #6618.

Fixes #4047.
Closes #6618.

src/cascadia/TerminalApp/defaults.json

@@ -187,15 +187,15 @@
             "foreground": "#839496",
             "background": "#002B36",
             "cursorColor": "#FFFFFF",
-            "black": "#073642",
+            "black": "#002B36",
             "red": "#DC322F",
             "green": "#859900",
             "yellow": "#B58900",
             "blue": "#268BD2",
             "purple": "#D33682",
             "cyan": "#2AA198",
             "white": "#EEE8D5",
-            "brightBlack": "#002B36",
+            "brightBlack": "#073642",
             "brightRed": "#CB4B16",
             "brightGreen": "#586E75",
             "brightYellow": "#657B83",
@@ -209,15 +209,15 @@
             "foreground": "#657B83",
             "background": "#FDF6E3",
             "cursorColor": "#002B36",
-            "black": "#073642",
+            "black": "#002B36",
             "red": "#DC322F",
             "green": "#859900",
             "yellow": "#B58900",
             "blue": "#268BD2",
             "purple": "#D33682",
             "cyan": "#2AA198",
             "white": "#EEE8D5",
-            "brightBlack": "#002B36",
+            "brightBlack": "#073642",
             "brightRed": "#CB4B16",
             "brightGreen": "#586E75",
             "brightYellow": "#657B83",