Update history.rst - no longer possible to build LTS images.

[marekm72]

Because LTS branches in the public repository are no longer updated, only rolling images can be built.

Checklist:

• I have read the CONTRIBUTING document

[nvollmar]

That's technically not correct, LTS branches can still be built. Just an additional step is needed to build the packages and setting up a apt repo.

[andamasov]

That's technically not correct, LTS branches can still be built. Just an additional step is needed to build the packages and setting up a apt repo.

Actually, we stopped updating public LTS branches
I will work with @rebortg to apply a wider update, not just history but also a page about building from source and some other pages. This will be after Stream published

[graybeard5x]

The current documentation is a bit misleading. Technically "LTS" can be built but will be missing updates. Perhaps this simple documentation change could be applied without waiting for Stream? That could help avoid situations where users think they are safe, when in fact they are running outdated "LTS" with unpatched security vulnerabilities. Thanks.