global: Use proper project name U-Boot (next2)

Use proper project name in README, rst and comment.
Done in connection to commit bb922ca3 ("global: Use proper project name
U-Boot (next)").

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <graf@csgraf.de> (ppce500)
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/536af05e7061982f15b668e87f941cdabfa25392.1694157084.git.michal.simek@amd.com