Add Heroku-20 to suggested stacks list

[edmorley]

Summary

Heroku-20 is now Heroku's default stack, and was previously added to the suggested builders list in #1052, however was not yet listed in the suggested stacks list.

See:
https://github.com/heroku/pack-images#heroku-pack-base-images

Output

Before

$ pack stack suggest
Stacks maintained by the community:

    Stack ID: heroku-18
    Description: The official Heroku stack based on Ubuntu 18.04
    Maintainer: Heroku
    Build Image: heroku/pack:18-build
    Run Image: heroku/pack:18

    Stack ID: io.buildpacks.stacks.bionic
    Description: A minimal Paketo stack based on Ubuntu 18.04
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:base-cnb
    Run Image: paketobuildpacks/run:base-cnb

    Stack ID: io.buildpacks.stacks.bionic
    Description: A large Paketo stack based on Ubuntu 18.04
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:full-cnb
    Run Image: paketobuildpacks/run:full-cnb

    Stack ID: io.paketo.stacks.tiny
    Description: A tiny Paketo stack based on Ubuntu 18.04, similar to distroless
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:tiny-cnb
    Run Image: paketobuildpacks/run:tiny-cnb

After

$ go run cmd/pack/main.go stack suggest
Stacks maintained by the community:

    Stack ID: heroku-18
    Description: The official Heroku stack based on Ubuntu 18.04
    Maintainer: Heroku
    Build Image: heroku/pack:18-build
    Run Image: heroku/pack:18

    Stack ID: heroku-20
    Description: The official Heroku stack based on Ubuntu 20.04
    Maintainer: Heroku
    Build Image: heroku/pack:20-build
    Run Image: heroku/pack:20

    Stack ID: io.buildpacks.stacks.bionic
    Description: A minimal Paketo stack based on Ubuntu 18.04
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:base-cnb
    Run Image: paketobuildpacks/run:base-cnb

    Stack ID: io.buildpacks.stacks.bionic
    Description: A large Paketo stack based on Ubuntu 18.04
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:full-cnb
    Run Image: paketobuildpacks/run:full-cnb

    Stack ID: io.paketo.stacks.tiny
    Description: A tiny Paketo stack based on Ubuntu 18.04, similar to distroless
    Maintainer: Paketo Project
    Build Image: paketobuildpacks/build:tiny-cnb
    Run Image: paketobuildpacks/run:tiny-cnb

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

[codecov]

Codecov Report

Merging #1122 (787d4aa) into main (aa49ae6) will decrease coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1122      +/-   ##
==========================================
- Coverage   80.54%   80.02%   -0.51%     
==========================================
  Files         136      136              
  Lines        8251     6010    -2241     
==========================================
- Hits         6645     4809    -1836     
+ Misses       1176      767     -409     
- Partials      430      434       +4     

Flag	Coverage Δ
os_linux	80.02% <ø> (+0.05%)	⬆️
os_macos	77.54% <ø> (ø)
os_windows	∅ <ø> (∅)
unit	80.02% <ø> (ø)

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