GraphQL: Add Project.topics and deprecate Project.tag_list

Introduction

• Related issue: #328226 (closed)
• Predecessor MR: !60834 (closed)

In GitLab, a project can have topics and tags (git tags) on its repository. Unfortunately, topics appear in various places under the name tags, which leads to a lot of confusion. As discussed in #328226 (closed), this confusion is to be cleared up. So I submitted the MR !60834 (closed) for that. However, the MR was too big. So I closed the MR and will submit several separate MRs instead.

What does this MR do?

This MR adds a new field topics to the Project type and deprecate the existing tag_list field.

Before:

query {project(fullPath: "gnuwget/Wget2") {name, tagList}}

{
    "data": {
        "project": {
            "name": "Wget2",
            "tagList": "topic1, topic2, topic3"
        }
    }
}

After:

query {project(fullPath: "gnuwget/Wget2") {name, topics}}

{
    "data": {
        "project": {
            "name": "Wget2",
            "topics": [
                "topic1",
                "topic2",
                "topic3"
            ]
        }
    }
}

🛠 with ❤ at Siemens

/cc @bufferoverflow

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