gh-35155: Update the logo at README.md
The original logo at the README.md seems broken while accessing from
GitHub's web interface. After careful consideration, I decided not to
use the original black logo (at
/src/doc/common/static/logo_sagemath_black.svg) due to potential
disparities in its visibility when viewed by users with different
display settings, such as light and dark modes. Therefore, I have update
it now to use different variants for dark and light schemes.
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URL: https://github.com/sagemath/sage/pull/35155
Reported by: Priyanshu Kumar Rai
Reviewer(s): Kwankyu Lee
