chore: update Go requirements and simplify go.mod

- Update minimum Go version from 1.21 to 1.23
- Remove toolchain directive for better compatibility
- Simplify go.mod for easier maintenance

Author: noborus

README.md

@@ -25,7 +25,7 @@ go get github.com/noborus/termhyo
 
 ### Requirements
 
-- Go 1.21 or later
+- Go 1.23 or later
 
 ## File Structure
 

go.mod

@@ -2,8 +2,6 @@ module github.com/noborus/termhyo
 
 go 1.23.0
 
-toolchain go1.24.6
-
 require (
 	github.com/mattn/go-runewidth v0.0.16
 	golang.org/x/text v0.28.0