[1.6.x] Fixed #21122 -- Improved clean up of test temp directory on Windows

- Fixed test that didn't close the files it opened
- Caught and handled exception when temp directory cannot be removed

Backport of 978e1351a6 of master
[1.6.x] Fixed #21218 -- Typo on docs/howto/upgrade-version.txt

Thanks ryan at ryangallen.com for the report.

Backport of 36e220f923 from master
[1.6.x] Fixed #21165 -- Fix test for syndication feed timestamp field on Windows.

Thanks Michael Manfre for the report, Raphaël Barrois for the patch and
Claude Paroz, Aymeric Augustin for the reviews.

Refs #7936

62dfd79f8b from master.
[1.6.x] Fixed #21235 -- Disabled savepoints for old versions of SQLite.

Thanks Ramiro for the report.

Backport of 91547772 from master.