various fixes vor php 8.1 compatibility

Cherry-picked from https://git-gitea.tt-rss.org/fox/tt-rss/pulls/56,
while excluding changes in vendor/ (causes compatiblity issues with
PHP<8 [1]) and strftime-related ones (already re-applied in
https://git-gitea.tt-rss.org/fox/tt-rss/pulls/66).

[1] https://community.tt-rss.org/t/support-for-php-8-1/5089/9