[JENKINS-48923] Core should use UTF-8 by default #3231
This started off as a follow-up to PR #3210 (see #3210 (comment) and PR #3224), but it appeared that fixing the one use of the default charset could be problematic, as there are several other uses of the default charset.
So, I have attempted to change the charset that Jenkins uses by default to UTF-8 across core.
This should be considered an experimental, work-in-progress PR.
Proposed changelog entries
Although I generally I agree with the change, it may have a serious impact on instances using another encoding, because they may improperly read previously saved files after the migration. It needs to be really well justified and tested if we want to merge it