Update Learn GitLab icon in navigation to bulb

What does this MR do and why?

The graduation hat was not in the SVG library and was not updated when the library had an overhaul to its stroke weight, which causes it to standout against the other items in the left navigation.

Screenshot

The change replaces the with to keep the icons in the left navigation a consistent stroke weight. It also removes the graduation_hat.svg from the repository.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run export GITLAB_SIMULATE_SAAS=1 before gdk start to simulate SaaS
2. Enable one of the feature flags: Feature.enable_percentage_of_time(:learn_gitlab_b_experiment_percentage, 100) # or the a variant
3. Signup with a new user
4. Go to the "Learn Gitlab" or any other project
5. Click on "Learn Gitlab"

Troubleshooting

Since we require the "learn-gitlab" project to be available, which gets created in a background-job, it could be that the project didn't get created. This is because background-jobs are sometimes stuck in development. What to do then:

1. gdk stop
2. ps aux | grep runsv , if there is any other process beside grep, you need to run kill -9 <pid>
3. gdk start again and go through the sign up flow again.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.