firefox-esr: checkver should use firefox_versions.json #16022
Description
https://wiki.mozilla.org/Release_Management/Product_details#firefox_versions.json
Product Details is a public JSON API which contains release information for Firefox.
The data provided by this API is meant to be the source of truth for all tools needing to know specific information about a Firefox release or channel.
In fact, firefox checkver already uses firefox_versions.json (#12690), firefox-esr should be consistent with firefox.
firefox_versions.json contains all the versions Mozilla ship for Firefox for Desktop.
{
"FIREFOX_AURORA": "",
"FIREFOX_DEVEDITION": "143.0b2",
"FIREFOX_ESR": "128.14.0esr",
"FIREFOX_ESR115": "115.27.0esr",
"FIREFOX_ESR_NEXT": "140.2.0esr",
"FIREFOX_NIGHTLY": "144.0a1",
"LAST_MERGE_DATE": "2025-08-18",
"LAST_RELEASE_DATE": "2025-08-19",
"LAST_SOFTFREEZE_DATE": "2025-08-14",
"LAST_STRINGFREEZE_DATE": "2025-08-15",
"LATEST_FIREFOX_DEVEL_VERSION": "143.0b2",
"LATEST_FIREFOX_OLDER_VERSION": "3.6.28",
"LATEST_FIREFOX_RELEASED_DEVEL_VERSION": "143.0b2",
"LATEST_FIREFOX_VERSION": "142.0",
"NEXT_MERGE_DATE": "2025-09-15",
"NEXT_RELEASE_DATE": "2025-09-16",
"NEXT_SOFTFREEZE_DATE": "2025-09-11",
"NEXT_STRINGFREEZE_DATE": "2025-09-12"
}