RTEMS | LICENSE.md: Add missing SLAC license and add introduction (#5200)

[Joel Sherrill (@joel)]

Joel Sherrill created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5200

Assignee: Gedare Bloom

## Summary

This issue is intended to address the following changes in LICENSE.md

- Add the missing SLAC License
- Move 2-BSD to the top
- Historical RTEMS license should be second
- Add introduction to mention relicensing, cite issue, and encourage relicensing if they haven't already 
- Add text clarifying that RTEMS only uses permissive licenses. Probably can link to Software Engineering Guide
- Verify that all licenses listed in [the old license page](https://web.archive.org/web/20220525045409/https://www.rtems.org/license) still in used are included


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