Fix version parsing for Paper 26+

[Xitee1]

Summary

• Bukkit.getBukkitVersion() on Paper 26.1.2 no longer matches the legacy <mc>-R<api>-SNAPSHOT format (it now looks like 26.1.2-build.41-alpha), so vstring.substring(0, vstring.lastIndexOf("-R")) produced a StringIndexOutOfBoundsException and aborted onEnable().
• Replace the substring parser with a regex that extracts the leading numeric version, tolerating arbitrary suffixes.
• Wrap the parser in a try/catch with a safe 1.20 fallback so a future format change can no longer prevent the plugin from enabling.

Mirrors the equivalent fix in PowerBoard (Xitee1/PowerBoard#76).

Fixes the crash:

java.lang.StringIndexOutOfBoundsException: Range [0, -1) out of bounds for length 21
  at de.xite.smp.main.Main.onEnable(Main.java:57)

Test plan

• Drop the built jar onto a Paper 26.1.2 server and confirm the plugin enables without StringIndexOutOfBoundsException
• Confirm Main.MCVersion is set to a numeric string like 26.1.2
• Confirm it still works on a Paper 1.20.x server

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