[Dev] use "main" branch as the default branch

[lisphilar]

Checkbox

• I agree to follow code of conduct and to engage in discussion actively.
• I have confirmed I will follow and update contribution guideline.

Issues of the current workflow

"main" branch is not used as the default branch.

Action items

Use "main" branch as the default branch.

Additional Context

No response

[lisphilar]

1. Rename the "master" branch to "main" branch and select "change default branch".
   https://github.com/lisphilar/covid19-sir/branches

2. Run the command in local environmant.

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

3. Update .github/workflows/**.yml to use "main" branch.