GHA/http3-linux: bump to ngtcp2 1.14.0, fix local build

GHA/http3-linux: bump to ngtcp2 1.14.0, fix local build

ngtcp2 1.14.0 added crypto dependencies to the ngtcp2 crypto `.pc`
files. It broke GHA builds, because how curl's `configure` is setting up
the per-dependency custom prefixes for pkg-config.

`configure` uses `PKG_CONFIG_LIBDIR` to set per-dependency custom
prefixes, as specified via `--with-ngtcp2=<custom-dir>`. In classic
`pkg-config` this overrides any previously configured `PKG_CONFIG_DIR`.
This in turn break detecting transitive pkg-config modules unless they
are found at locations `pkg-config` is searching by default. This
doesn't affect `pkgconf` because it appends `PKG_CONFIG_LIBDIR` to
the custom `PKG_CONFIG_DIR`, according to its man page.

It may make sense to fix this in`acinclude.m4`, to make sure to honor
global custom pkg-config paths while detecting components at custom
locations, regardless of pkg-config implementation. But this PR doesn't
do this.

Instead it drops the ngtcp2 custom path and lets detection rely on
`PKG_CONFIG_DIR` that's already set up for all custom-built dependencies
anyway.

Also:
- fix `openssl-quic` job to use the custom-built nghttp2 (like other
  jobs do) instead of the system default.
- configure nghttp3 via `PKG_CONFIG_DIR` in the `openssl-quic` job,
  to sync with other jobs. And drop `--with-nghttp3` option.

https://github.com/curl/curl/blob/cb9b1a4c4e875ac4eac2209d8686acef3114abdf/acinclude.m4#L1376-L1381
https://manpages.debian.org/unstable/pkg-config/pkg-config.1.en.html
https://man.archlinux.org/man/pkgconf.1.en

Ref: https://github.com/ngtcp2/ngtcp2/pull/1689#issuecomment-3121576712
Closes #18022
Closes #18028