Update the "Fetch icons" workflow

[alizedebray]

• If an icon is removed, the change set should mark the change as major;
• If an icon is added, the change set should mark the change as minor;
• If an icon is updated, the change set should mark the change as a fix;
• The change set description is currently always plural, it must be singular if only one icon has changed.
• There are missing spaces in the changelog.

[oliverschuerch]

Ask @alizedebray for more details, before you start working on this issue.

[alizedebray]

Done with https://github.com/swisspost/design-system/pull/2453