Schema: only require Repo, RepoType and SourceCode

Also fix all schema errors.

The NoSourceSince are added by a script

import yaml
from yaml import SafeLoader
from pathlib import Path

for file in Path(".").glob("metadata/*.yml"):
    with file.open("r", encoding="utf-8") as f:
        data = yaml.load(f, Loader=SafeLoader)
    if not data.get("NoSourceSince") and not data.get("Repo"):
        version = data.get("CurrentVersion")
        if version:
            with file.open("a") as f:
                f.write(f"\nNoSourceSince: {version}\n")

and then run fdroid rewritemeta.

Disable auto update when the app is discontinued.

import yaml
from yaml import SafeLoader
from pathlib import Path
import re

subs = [
    (r"AutoUpdateMode: .*", "AutoUpdateMode: None"),
    (r"UpdateCheckMode: .*", "UpdateCheckMode: None"),
]

for file in Path(".").glob("metadata/*.yml"):
    with file.open("r", encoding="utf-8") as f:
        data = yaml.load(f, Loader=SafeLoader)
    if data and (data.get("NoSourceSince") or data.get("ArchivePolicy") == "0 versions"):
        text = file.read_text()
        for old, new in subs:
            text = re.sub(old, new, text)
        file.write_text(text)

@FestplattenSchnitzel @licaon-kter @IzzySoft

Closes #2841 (closed)