Change: replace per-AI "start_date" with a global "competitors_interval" (#10653)

The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.

While at it, also ported this part over to the new timer system.

src/saveload/saveload.h

@@ -351,6 +351,7 @@ enum SaveLoadVersion : uint16 {
 	SLV_CONSISTENT_PARTIAL_Z,               ///< 306  PR#10570 Conversion from an inconsistent partial Z calculation for slopes, to one that is (more) consistent.
 	SLV_MORE_CARGO_AGE,                     ///< 307  PR#10596 Track cargo age for a longer period.
 	SLV_LINKGRAPH_SECONDS,                  ///< 308  PR#10610 Store linkgraph update intervals in seconds instead of days.
+	SLV_AI_START_DATE,                      ///< 309  PR#10653 Removal of individual AI start dates and added a generic one.
 
 	SL_MAX_VERSION,                         ///< Highest possible saveload version
 };