Project name is case-sensitive on `down` but not other commmands

[haines]

Description

If I run docker-compose -p SomethingWithMixedCase, the project name is downcased for most commands, but not docker-compose down.

Steps to reproduce the issue:

1. Create docker-compose.yml with any valid compose file, e.g.

   services:
     hello-world:
       image: hello-world

2. Create the resources with up or run using a project name with mixed case, e.g.

   $ docker-compose -p HelloWorld run hello-world
   [+] Running 1/1
    ⠿ Network helloworld_default  Create...                              0.1s

   Note that it downcases the project name.

3. Attempt to remove the resources with down using the same project name, e.g.

   $ docker-compose -p HelloWorld down

Describe the results you received:

[+] Running 1/0
 ⠿ HelloWorld  Warning: No resource found to remove

Describe the results you expected:

[+] Running 2/2
 ⠿ Container helloworld_hello-world_1  Removed                        0.1s
 ⠿ Network helloworld_default          Removed                        0.1s

Additional information you deem important (e.g. issue happens only occasionally):

This works as expected in docker-compose 1.x.

Output of docker-compose --version:

Docker Compose version v2.0.0-rc.1

Output of docker version:

Client:
 Cloud integration: 1.0.17
 Version:           20.10.8
 API version:       1.41
 Go version:        go1.16.6
 Git commit:        3967b7d
 Built:             Fri Jul 30 19:55:20 2021
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.8
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.6
  Git commit:       75249d8
  Built:            Fri Jul 30 19:52:10 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.9
  GitCommit:        e25210fe30a0a703442421b0f60afac609f950a3
 runc:
  Version:          1.0.1
  GitCommit:        v1.0.1-0-g4144b63
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0