[4.0] [plg_privacy_message] Convert to prepared statement

[HLeithner]

Summary of Changes

Updated SQL queries to prepared statements and made some cleanups around the queries. This PR needs joomla-framework/database#162

Testing Instructions

Use the plugin in all ways you can think of.

Create users
Generate privacy message entries.

Expected result

Nothing changed.

[brianteeman]

Looks like there are two unrelated files here

[HLeithner]

thx, I will fix all 3 PR later

[HLeithner]

Removed the orWhere function based discussion joomla-framework/database#174

[SharkyKZ]

I have tested this item ✅ successfully on 427423b

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[Quy]

I have tested this item ✅ successfully on 427423b

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[Quy]

RTC

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[wilsonge]

Thanks!