Fix gradio version range to allow SDK 6.x

HF Spaces injects gradio==6.8.0 from the README frontmatter,
which conflicted with the >=5,<6 constraint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements.txt

@@ -1,6 +1,6 @@
 # App
 accelerate>=1,<2
-gradio>=5,<6
+gradio>=5,<7
 sentencepiece>=0.2,<1
 spaces>=0.34,<1
 tokenizers>=0.21,<1