Cache is_multisite() and is_network_admin() to reduce redundant calls

[mukesh27]

The wp-admin/plugins.php file makes repeated calls to is_multisite() and is_network_admin() throughout the plugin management logic. This leads to redundant function calls that can add up on sites with many plugins.

Trac ticket: https://core.trac.wordpress.org/ticket/64077

This PR refactors the src/wp-admin/plugins.php file to improve efficiency and maintainability by caching the results of is_multisite() and is_network_admin() in local variables. All subsequent checks throughout the file now use these cached variables, reducing repeated function calls and improving readability. No functional behavior is changed.

Refactoring for efficiency and readability:

• Cached is_multisite() and is_network_admin() results in $is_multisite and $is_network_admin variables at the top of the file, and replaced all repeated calls to these functions with the variables throughout the plugin management logic.
• Updated all conditional checks and function calls that previously used is_multisite() and is_network_admin() to use the new local variables, including plugin activation, deactivation, automatic updates, and UI display logic.

[mukesh27]

As discussed in PR we don't need these changes for now.