chore: replace immutable.Seq with Seq, drop Scala 2.12-era imports

[pjfanning]

Motivation

The codebase still carried the Scala 2.12-era immutable.Seq qualifier across ~100 files. scala.Seq has been a type alias for scala.collection.immutable.Seq since 2.13, so the qualifier — and the scala.collection.immutable imports that exist only to support it — are redundant. This is the pekko-http counterpart of apache/pekko#3539.

Modification

• Replace immutable.Seq, collection.immutable.Seq and scala.collection.immutable.Seq with Seq across main, test, multi-jvm and docs-snippet Scala sources (100 files).
• Drop import scala.collection.immutable (70 files) and import scala.collection.immutable.Seq (13 files) where no other immutable.* member is used; the import is kept in the 16 files that still reference immutable.TreeMap, VectorBuilder, ListMap, IndexedSeq, etc.
• Http2Blueprint: import scala.collection.{ immutable, mutable } -> import scala.collection.mutable.
• Paradox docs (6 files): prose and signature mentions updated from immutable.Seq to Seq; the alphabetically.md table rows are re-padded so the columns stay aligned.
• Java sources (Util.java, MediaTypes.java, RegexConverters.java) are deliberately unchanged: scala.Seq is a package-object alias that Java cannot name, so they must keep scala.collection.immutable.Seq.

The other idioms cleaned up in apache/pekko#3539 (WrappedArray, filterKeys/mapValues, .toIterator, Stream, .left.get/.right.get) do not occur in this repo — the existing mapValues/filterKeys calls already go through .view.

Result

No remaining immutable.Seq idioms in Scala sources or docs. The change is source- and binary-compatible (identical erasure); unlike the pekko PR, no MiMa filter was needed.

Tests

• native scalafmt 3.7.17 run on all changed Scala files
• sbt Test/compile multi-jvm:compile passes on Scala 2.13.18 (Scala 3 left to CI)
• sbt http-core/mimaReportBinaryIssues http/mimaReportBinaryIssues http-cors/mimaReportBinaryIssues http-xml/mimaReportBinaryIssues — all pass against 1.0.0 with mimaReportSignatureProblems := true (testkit has MiMa disabled in the build)
• sbt http-cors/test plus HeaderSpec, HttpHeaderSpec, FramingSpec, Http2FramingSpec, HttpMessageRenderingSpec and the two Http*DetailedStringExampleSpec doc specs — 201 tests, 0 failures
• No directional test added: pure type-alias refactor with no behavioural change; the existing suites are the directional coverage

References

None - follows apache/pekko#3539