docs: update README banner assets

Author: jikkai

README.md

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 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="./docs/img/banner-light.png">
-    <img src="./docs/img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="./docs/img/banner.png" alt="Univer SDK" width="800" />
 
 **A full-stack, isomorphic office SDK for building spreadsheets, documents, and presentations.**
 

docs/img/banner-dark.png

@@ -1,9 +1,6 @@
 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="../img/banner-light.png">
-    <img src="../img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="../img/banner.png" alt="Univer SDK" width="800" />
 
 **Un SDK ofimático full-stack e isomórfico para crear hojas de cálculo, documentos y presentaciones.**
 

docs/img/banner-light.png

@@ -1,9 +1,6 @@
 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="../img/banner-light.png">
-    <img src="../img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="../img/banner.png" alt="Univer SDK" width="800" />
 
 **スプレッドシート、ドキュメント、プレゼンテーションを構築するためのフルスタックでアイソモーフィックなオフィス SDK。**
 

docs/img/banner.png

@@ -1,9 +1,6 @@
 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="../img/banner-light.png">
-    <img src="../img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="../img/banner.png" alt="Univer SDK" width="800" />
 
 **스프레드시트, 문서, 프레젠테이션을 만들기 위한 풀스택 동형 오피스 SDK.**
 

docs/readme/es-ES.md

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 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="../img/banner-light.png">
-    <img src="../img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="../img/banner.png" alt="Univer SDK" width="800" />
 
 **用于构建电子表格、文档和演示文稿的全栈、同构办公 SDK。**
 

docs/readme/ja-JP.md

@@ -1,9 +1,6 @@
 <div align="center">
 
-<picture>
-    <source media="(prefers-color-scheme: dark)" srcset="../img/banner-light.png">
-    <img src="../img/banner-dark.png" alt="Univer" width="420" />
-</picture>
+<img src="../img/banner.png" alt="Univer SDK" width="800" />
 
 **用於構建試算表、文件和簡報的全端、同構辦公 SDK。**