GHA: unify http3 workflows into one

GHA: unify http3 workflows into one

This commit unifies the following http3 workflows into http3-linux.yml:

- ngtcp2-linux.yml
- osslq-linux.yml
- quiche-linux.yml

The idea is better use of the build cache. Previously, they
independently create caches with the same key. Some of the caches
include source code and intermediate object files, which makes cache
quite large. In this commit, only built artifacts are cached, which
drastically reduces the cache size. OpenSSL v3, mod_h2 and quiche caches
still include all stuff, but they are left for the later improvement.
Because the contents of the cache have been changed, the cache keys are
also changed to include the word "http3".

Closes #13841