Upgrade nodejs apt source repo from 18.x to 20.x

Required version is updated from v18.16.x to v20.13.x
with GitLab v17.0.0 release.

Documentation:
- "Bump required Node.js version to 20 in installation guide"
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/154523

.tool-versions:
- "Update .tool-versions and .nvmrc to use nodejs v20.12.2"
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149615

- Set distro to "nodistro" for nodejs to get later updates

GitLab v17.0.0 or later requires (grants) nodejs v20.13.x or later,
but nodejs v20.5.1 is installed for focal even later LTS are released.
To avoid this issue, official installer script uses "nodistro"
instead of specific distribution.

See:
https://github.com/nodesource/distributions/blob/83867e0fdde8ebb17f5f83644f00c4bb180a4568/scripts/deb/setup_20.x#L74

Author: kkimurak

Dockerfile

@@ -37,7 +37,7 @@ RUN set -ex && \
  && wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add - \
  && echo 'deb http://apt.postgresql.org/pub/repos/apt/ focal-pgdg main' > /etc/apt/sources.list.d/pgdg.list \
  && wget --quiet -O - https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - \
- && echo 'deb https://deb.nodesource.com/node_18.x focal main' > /etc/apt/sources.list.d/nodesource.list \
+ && echo 'deb https://deb.nodesource.com/node_20.x nodistro main' > /etc/apt/sources.list.d/nodesource.list \
  && wget --quiet -O - https://dl.yarnpkg.com/debian/pubkey.gpg  | apt-key add - \
  && echo 'deb https://dl.yarnpkg.com/debian/ stable main' > /etc/apt/sources.list.d/yarn.list \
  && set -ex \