Fix FIRRTL spec emission and bump to spec 1.2.0 (backport #3094) (#3097)

* Fix FIRRTL spec emission and bump to spec 1.2.0 (#3094)

* Emit FIRRTL spec version in Emitter
* Bump FIRRTL spec version to 1.2.0

(cherry picked from commit 1ef1fc4e

)

# Conflicts:
#	firrtl/src/main/scala/firrtl/ir/Serializer.scala
#	firrtl/src/test/scala/firrtlTests/ExtModuleTests.scala
#	src/test/scala/circtTests/stage/ChiselStageSpec.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>