build(docs-infra): upgrade preview server Docker image to Debian 10 (buster) (#36837)
build(docs-infra): upgrade preview server Docker image to Debian 10 (buster) (#36837)

Previously, the preview server Docker image was based on Debian 9
(stretch).

This commit upgrades the preview server Docker image to Debian 10
(buster) and also upgrades all dependencies to latest versions
(including upgrading Node.js from v10 to v12).

(The GCE VM running the preview server Docker container was also
upgraded from Debian 9 to 10 on 2020-04-27.)

---
Other changes:
- Pinned the installed version of `curl` to make the `aio-health-check`
  and `aio-verify-setup` checks (which use `curl`) more deterministic.
- Dropped the `*-backports` Debian repositories, since they are no
  longer needed. The `*-backports` repositories were introduced in
  593fe5ed259d5b118d41e2b961b462e922762700 to install `nginx` from, but
  became obsolete in 2f1a862b831f44b9b923dcc52f94af69d3b40aa6, which
  switched to installing `nginx` from the regular repositories again.
- Added `vim` to the list of installed dependencies (for convenience
  during debugging).

PR Close #36837

ocombe/angular,angular-indonesia/angular,blesh/angular,angular/angular,IdeaBlade/angular,ocombe/angular,filipesilva/angular,mgechev/angular,blesh/angular,filipesilva/angular,gnomeontherun/angular,blesh/angular,alxhub/angular,ocombe/angular,mgechev/angular,gdi2290/angular,gkalpak/angular,wKoza/angular,JiaLiPassion/angular,ocombe/angular,blesh/angular,alxhub/angular,manekinekko/angular,mgechev/angular,IdeaBlade/angular,wKoza/angular,manekinekko/angular,alxhub/angular,angular-indonesia/angular,brandonroberts/angular,manekinekko/angular,gkalpak/angular,IdeaBlade/angular,shairez/angular,blesh/angular,alxhub/angular,angular/angular,ocombe/angular,wKoza/angular,ocombe/angular,wKoza/angular,brandonroberts/angular,JiaLiPassion/angular,gdi2290/angular,gnomeontherun/angular,brandonroberts/angular,manekinekko/angular,angular/angular,gdi2290/angular,mgechev/angular,manekinekko/angular,filipesilva/angular,gdi2290/angular,gnomeontherun/angular,shairez/angular,gkalpak/angular,IdeaBlade/angular,gdi2290/angular,gnomeontherun/angular,brandonroberts/angular,brandonroberts/angular,manekinekko/angular,angular-indonesia/angular,gkalpak/angular,filipesilva/angular,wKoza/angular,shairez/angular,angular-indonesia/angular,wKoza/angular,filipesilva/angular,gdi2290/angular,JiaLiPassion/angular,shairez/angular,angular/angular,alxhub/angular,mgechev/angular,IdeaBlade/angular,brandonroberts/angular,shairez/angular,JiaLiPassion/angular