Add deprecated project filter component

What does this MR do?

On the vulnerability report page, there is a collection of filter dropdowns:

Each dropdown is either a simple filter, or a variant that extends it. Because simple_filter.vue is used/extended for multiple filters, for flexibility the original implementation added the ability (through props) to show a search box and show a loading spinner. Since then though, it's become apparent that only the project filter needs to show a search box, and the loading prop is not used at all.

This MR refactors this by doing the following:

1. The unused loading prop is removed from simple_filter.vue. Any filter that needs to use this prop will be a filter that extends simple_filter.vue.
2. A new project_filter_old.vue component is created, and the code for showing the search box is moved into it from simple_filter.vue. It's called project_filter_old.vue because we have a newer project_filter.vue that's currently under testing, and will eventually replace project_filter_old.vue.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.