Drop 2.12 support, add scalafmt, scalafix, MiMa

[migesok]

Source and binary compat preserved.

The code changes:

• scalafmt and scalafix formatting applied
• added type annotations to public members and implicits
• migrated to Scala 3 syntax for imports
• since 2.12 is not supported anymore, replaced CurrentThreadExecutionContext wrapper with direct usage of ExecutionContext.parasitic. This as well leads to removal of the direct dependency on executor-tools as it is not used in the project anymore.

[coveralls]

Pull Request Test Coverage Report for Build 10680242506

Details

• 15 of 22 (68.18%) changed or added relevant lines in 8 files are covered.
• 14 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.7%) to 85.514%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sequentially-metrics/src/main/scala/com/evolutiongaming/concurrent/SequentiallyMetrics.scala	0	1	0.0%
sequentially/src/main/scala/com/evolutiongaming/concurrent/sequentially/SequentiallyAsync.scala	2	3	66.67%
sequentially-metrics/src/main/scala/com/evolutiongaming/concurrent/MeteredSequentiallyAsync.scala	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
sequentially/src/main/scala/com/evolutiongaming/concurrent/sequentially/Sequentially.scala	1	92.86%
benchmark/src/main/scala/com/evolutiongaming/concurrent/sequentially/SequentiallyBenchmark.scala	1	0.0%
sequentially-metrics/src/main/scala/com/evolutiongaming/concurrent/MeteredSequentiallyAsync.scala	1	0.0%
benchmark/src/main/scala/com/evolutiongaming/concurrent/sequentially/SequentiallyHandlerBenchmark.scala	2	0.0%
benchmark/src/main/scala/com/evolutiongaming/concurrent/sequentially/SequentiallyAsyncBenchmark.scala	2	0.0%
benchmark/src/main/scala/com/evolutiongaming/concurrent/sequentially/SequentiallyStreamBenchmark.scala	2	0.0%
sequentially-metrics/src/main/scala/com/evolutiongaming/concurrent/SequentiallyMetrics.scala	5	0.0%

Totals
Change from base Build 9446050682:	0.7%
Covered Lines:	183
Relevant Lines:	214

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💛 - Coveralls