Pin highest supported PHP version in `.env` for 6.5 branch

[desrosj]

When a fresh branch for a new release is created, the value of LOCAL_PHP in the .env file should be changed from latest (or whatever the value in trunk was at the time) to the highest version of PHP supported by that version.

This prevents test workflows and the local development environment from breaking in the future when the container tagged latest changes to a newer version of PHP.

It looks like this was missed during the 6.5 branching.

Previously #60095.

[desrosj]

In 58253:

Build/Test Tools: Pin PHP 8.3 to LOCAL_PHP in the 6.5 branch.

This ensures the local development environment and testing workflows do not break in the future when the PHP Docker container labeled latest changes.

Fixes #61329.
See #60095.