docs(readme): fix broken links and update Go version prerequisite

[Napat]

• Fix the broken link for Auth Middleware to point to the correct URL /advanced-guide/authentication instead of /advanced-guide/http-authentication.
• Add a hyperlink for Custom Middleware pointing to /advanced-guide/middlewares.
• Update the Go version prerequisite from 1.24 to 1.26 to align with the strict toolchain requirement defined in go.mod.

Pull Request Template

Description:

This pull request addresses three issues in the README.md to improve documentation accuracy and maintain alignment with the framework's configuration.

Additional Information:

1. Fix Auth Middleware URL: The previous link (https://gofr.dev/docs/advanced-guide/http-authentication) was broken due to a directory restructuring. It has been updated to the correct active path: https://gofr.dev/docs/advanced-guide/authentication (aligning with docs/navigation.js).
2. Add Custom Middleware URL: Added a direct link to the Custom Middleware documentation (https://gofr.dev/docs/advanced-guide/middlewares) to make the "Custom Middleware Support" text interactive and easy to access.
3. Align Go Prerequisite Version: The README.md previously stated that Go 1.24 or above is required. However, the go.mod file strictly specifies go 1.26.0. Under Go 1.21+ behavior, the Go compiler strictly enforces the go directive in go.mod as a hard minimum. Therefore, the Go prerequisite in README.md has been updated to 1.26 to align with the framework's real build requirements.

• Documentation update (non-breaking change which fixes or improves documentation)

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.