Simplify remembered flags deprecation message

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

This deprecation message recommends bundle config set --local, however bundle config is now local by default. It also quotes some values unnecessarily.

What is your fix for the problem, implemented in this PR?

Simplify the deprecation message.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[deivid-rodriguez]

The "risk" of this is that users may copy the deprecation message to an app still using a old Bundler version where bundle config was not local, so maybe not worth changing.

[indirect]

I think it is fine to print messages that only apply to the running version of Bundler, but thank you for calling out that risk!