Tvheadend

While trying to fix the CI, I see jessie and stretch builds are causing issues when doing `apt-get update` because the mirrors no longer carry the files.

Debian does offer snapshot/archives, and doing something like:

```
--- a/.github/workflows/build.yml
++ b/.github/workflows/build.yml
@ -154,6 +154,14 @ jobs:
container:
image: ${{ matrix.container }}
steps:
- name: Fix legacy versions
+ if: |
+ contains(matrix.container, 'jessie') ||
+ contains(matrix.container, 'stretch')
+ run: |
+ sed -i \
+ -e 's|\(deb\(-src\)\?\)\s\+\([hf]t\{1,2\}p\)|\1 [check-valid-until=no] \3|' \
+ -e '/snapshot/s/^#:space:*\(\(.*snapshot.*\)*\)/\1/; /snapshot/! s/^/# /' /etc/apt/sources.list
- name: Add git ppa
if: startsWith(matrix.container, 'ubuntu') || startsWith(matrix.container, 'i386/ubuntu')
run: |
```

(needs a bit more on the apt-get install line too, because of expired signatures)

is a work around, but it's also very slow, hammering the debian archive servers.

So before contiuing with this solution, I ask you, is it worth-while to drop jessie and stretch from the CI?
~
If users create bug reports, sure, we can consider fixing them of course. But jessie and stretch are ~10 years old, run on 3.19 and 4.9 kernels, security updates stopped 3+ years ago, LTS stopped last year and 3 years ago and only 'feexian ELTS', whatever that may be lasts for a nother 5 years.

So lets drop it yeah?