build: migrate to sbt 1.x and cross-publish for Scala 2.13

[hongwei1]

Why

OBP-API consumes this repository through JitPack as com.github.OpenBankProject:scala-macros:v1.0.0-alpha.3 and is being migrated to Scala 2.13. That aggregate pom hardcodes 2.12 artifacts:

<artifactId>core_2.12</artifactId>
<artifactId>macros_2.12</artifactId>
<artifactId>scala-macros_2.12</artifactId>

An already-published pom cannot change, so a 2.13 build has to come from a new tag. Two things block that today: the build definition is sbt 0.13 syntax pinned to a single Scala version, and JitPack's default sbt command publishes only the default Scala version.

What

• project/build.properties: sbt 0.13.11 -> 1.10.11.
• build.sbt: drop the 0.13-only run <<= run in Compile in core root-run delegation, move scalaVersion in ThisBuild to sbt 1.x slash syntax, and cross-build 2.12.21 + 2.13.18 (the versions OBP-API compiles with).
• project/plugins.sbt, project/assembly.sbt: removed. scalajs, sonatype, pgp, release and assembly are sbt 0.13-era plugins that no setting in this build references, and they block the sbt 1.x upgrade.
• jitpack.yml: override JitPack's clean publishM2 with +publishM2 so a single tag publishes both the _2.12 and the _2.13 artifacts.

No source file changed. The macro is a blackbox def macro rather than a macro annotation, so it does not need the paradise plugin, and the reflection API it uses (c.internal.enclosingOwner, paramLists, c.freshName, quasiquotes) is unchanged in 2.13.

Verification

sbt +clean +compile +publishM2 on JDK 25, zero [error] lines, both versions compiled and published:

Scala	macros	core	Published
2.12.21	4 sources	2 sources	scala-macros_2.12, macros_2.12, core_2.12
2.13.18	4 sources	2 sources	scala-macros_2.13, macros_2.13, core_2.13

After a tag is cut, the published artifacts must be verified by fetching pom and jar directly rather than trusting JitPack's maven-metadata.xml, which lists versions that do not exist.