make XmlSerializer's properties accessable

[jkowalleck]

XmlSerializer's properties xmlVersion and xmlEncoding are currently private.
goal: make them visible/accessible for inheriting structures.
caused by #304 (reply in thread)

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options for a solution:

• a) add protected getters
• b) make properties protected readonly -- since this lib requires php:^8.1

preferred solution: b)

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this is considered a non-breaking change, as an access is widened.

[jkowalleck]

@llaville FYI

[llaville]

preferred solution b) too !