Ubuntu 18.04 reached end-of-life on 31 - May - 2023

Ubuntu 18.04 reaches end of life on May 31, 2023
This issue is created to track progress for the remove support for Ubuntu 18.04 from .NET 6.0 and .NET 7.0

### Repos team to update:
- [ ] Remove installers for the outdated distro
- [ ] Remove build support
- [ ] Remove test assets

### Infrastructure Team Accountabilities/Deliverables:
- [ ] Notification to NetCore Engineering Partners
- [ ] Remove the outdated version from the hardware pool

### Release Team
- [ ] Send Notification to every team in .NET for upcoming addition
- [ ] Supported OS content gets updated on GitHub and marketing sites.
- [ ] Update release manifest files.
- [ ] Docker files which are used for install testing needs to be updated.

### Document updates:
- [ ] Add/Remove OS prerequisites to the dependencies document
- [ ] Update distro install page

/CC @rbhanda Created for your convenience since I just updated our internal docs anyway. os-support label and assignees are missing :)