Use date of last posting to determine month

[Nef10]

Sheets often include forgotten postings from prior month,
therefore it is better to use the month of the latest posting
to determine the month of the sheet

[github-actions]

Coverage after merging month into main

0.00%

Coverage Report

File	Stmts	Funcs	Lines	Uncovered Lines
/Users/runner/work/SwiftBeanCountSheetSync/SwiftBeanCountSheetSync/Sources/SwiftBeanCountSheetSync
Downloader.swift	0%	0%	0%	16–38, 40–54, 56–68
LedgerReader.swift	0%	0%	0%	15–22, 24–65
LedgerSettings.swift	0%	0%	0%	62–70
SheetDownloader.swift	0%	0%	0%	19–26, 29–33, 35–51
SheetParser.swift	0%	0%	0%	100–115, 117–123, 125–146, 20–29, 50–54, 56–75, 77–99
Syncer.swift	0%	0%	0%	100–102, 104–113, 119–121, 123–125, 127–129, 131–137, 140–158, 27–36, 57–62, 77–80, 83–89, 91–95, 97–99
TransactionMapper.swift	0%	0%	0%	13–15, 17–39
Uploader.swift	0%	0%	0%	16–37, 43–47