Follow #75 to update all variables (#135)

Reviewed-on: https://gitea.com/gitea/docs/pulls/135
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>

versioned_docs/version-1.21/usage/actions/variables.md

@@ -25,7 +25,7 @@ The following rules apply to variable names:
 - Variable names must not start with a number.
 - Variable names are case-insensitive.
 - Variable names must be unique at the level they are created at.
-- Variable names must not be `CI`.
+- Variable names must not start with `CI`.
 
 ## Using variable
 

versioned_docs/version-1.22/usage/actions/variables.md

@@ -19,7 +19,7 @@ The following rules apply to variable names:
 - Variable names must not start with a number.
 - Variable names are case-insensitive.
 - Variable names must be unique at the level they are created at.
-- Variable names must not be `CI`.
+- Variable names must not start with `CI`.
 
 ## Using variable
 

versioned_docs/version-1.23/usage/actions/variables.md

@@ -19,7 +19,7 @@ The following rules apply to variable names:
 - Variable names must not start with a number.
 - Variable names are case-insensitive.
 - Variable names must be unique at the level they are created at.
-- Variable names must not be `CI`.
+- Variable names must not start with `CI`.
 
 ## Using variable