Adjustments for deprecation of @calendar

[raffazizzi]

To be discussed at the next council meeting because we need to decide whether to do Step 0 below which would cause a delay to the deprecation period.

Optional Step 0:

• announce that there will be an upcoming change for ODD writers who want to include @calendar via att.datable, because:

Step 1:

• make date, docDate, origDate, and time members of att.calendarSystem
• remove @calendar from att.datable
• define @calendar on the 40+ elements that it needs to be removed from and provide @validUntil on those attributes. This will make sure that the deprecation notice appears on the guidelines as well as in the generated schema.

Step 2:

• complete process by removing @calendar from the elements.

[raffazizzi]

During the 2023-10/13 council call we decided to:

• proceed with Step 1 on this branch to be merged into dev when ready;
• warn ODD users on TEI-L about the forthcoming change (likely coming in the November 23 release)
• open new ticket to discuss deprecation of changes that affect ODD writing even when they don't directly affect the generated TEI schema. Another example can be found in the previous release (4.6.0): "The model.glossLike class (which contained altIdent, equiv, and gloss) has been renamed to model.identSynonyms."