Update PRODID and VERSION property handling

[axunonb]

Issue: Calendar has setters for ProductId and Version which are overridden with fixed values when serializing.

• When creating a new Calendar instance, ProductId and Version contain default values
• When Deserializing an iCalendar, ProductId and Version will be taken from the input
• ProductId and Version can be overridden by user code. An attempt to set as an empty string will throw.
• Update the default PRODID property LibraryMetadata.ProdId to include the ical.net assembly version. Example: "PRODID:-//github.com/ical-org/ical.net//NONSGML ical.net 5.4.3//EN"
• Modified CalendarSerializer.SerializeToString so that the ProdId or Version set by users do not get overridden
• Add an xmldoc description about the purpose of ProdId and Version, and about the risks when modified
• Add unit tests

Resolves #531

[axunonb]

@minichma TBD: Add the current ical.net version to the PRODID

[minichma]

The RFC says the PRODID should contain

Any text that describes the product and version

so, yes, including the version would probably be a good idea.

[axunonb]

@minichma RFC5545 says that PRODID and VERSION are mandatory. This may be the reason for the current implementation (always set the defaults when serializing).
But given this ics with missing PRODID and VERSION to deserialize:

BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART:20070406T230000Z
DTEND:20070407T010000Z
END:VEVENT
END:VCALENDAR

TBD:
Options

• Make it compliant by adding missing values: Then we don't see the original values.
• Leave it invalid. It's up to the user to fix, if needed before serializing (my preferred option, - breaking change to previous versions)
• Leave the properties unchanged (invalid), but replace empty PRODID or VERSION when serializing (non-braking, but kind of unexpected magic)

[minichma]

Leave it invalid. It's up to the user to fix, if needed before serializing (my preferred option, - breaking change to previous versions)

My preferred option too. Only populate ProdId and Version when creating a new calendar, but not when deserializing an existing one.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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