Remove deprecated ioutil and unused function

[lucacome]

This pull request includes several changes to the main.go file, focusing on code cleanup and improvements to file handling. The most important changes include removing an unused function, replacing deprecated functions, and improving code readability.

Code cleanup and improvements:

• Removed the unused getenvInt function.
• Replaced deprecated ioutil.ReadFile and ioutil.WriteFile with os.ReadFile and os.WriteFile respectively for better performance and future compatibility. [1] [2]
• Consolidated the os.OpenFile call into a single line for improved readability.

File handling improvements:

• Updated the setGithubEnvOutput function to use fmt.Fprintf for writing to the file, enhancing clarity and performance.

[jacobtomlinson]

These changes look great. Many thans!