Revert "stack.yaml - switch to lts-15."

This reverts commit a238128a8e.

This version fails on Windows.  We get the following log output:

Stack has not been tested with GHC versions above 8.6, and using 8.8.2, this may fail
Stack has not been tested with Cabal versions above 2.4, but version 3.0.1.0 was found, this may fail
Building all executables for `pandoc' once. After a successful build of all of them, only specified executables will be rebuilt.
pandoc> configure (lib + exe + test)
Configuring pandoc-2.9.2...
pandoc> build (lib + exe + test)
Preprocessing library for pandoc-2.9.2..
Building library for pandoc-2.9.2..
[  1 of 162] Compiling Paths_pandoc
[  2 of 162] Compiling Text.Pandoc.App.FormatHeuristics
[  3 of 162] Compiling Text.Pandoc.Asciify
[  4 of 162] Compiling Text.Pandoc.CSV
[  5 of 162] Compiling Text.Pandoc.Data
C:\Users\runneradmin\AppData\Roaming\stack\snapshots\9b7c3910\lib\x86_64-windows-ghc-8.8.2\texmath-0.12.0.1-LHa5Nt7Y9k12Dq03wVcYbb\HStexmath-0.12.0.1-LHa5Nt7Y9k12Dq03wVcYbb.o: unhandled PEi386 relocation type 0
ghc.exe: unable to load package `texmath-0.12.0.1'
      C:\Users\runneradmin\AppData\Roaming\stack\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.2.exe --builddir=.stack-work\dist\29cc6475 build lib:pandoc exe:pandoc test:test-pandoc --ghc-options " -fdiagnostics-color=always"

stack.yaml

@@ -12,9 +12,19 @@ flags:
 packages:
 - '.'
 extra-deps:
+- pandoc-types-1.20
+- texmath-0.12.0.1
+- haddock-library-1.8.0
+- skylighting-0.8.3.2
+- skylighting-core-0.8.3.2
+- regex-pcre-builtin-0.95.0.8.8.35
 - doclayout-0.3
+- emojis-0.1
+- jira-wiki-markup-1.0.0
+- HsYAML-0.2.0.0
+- HsYAML-aeson-0.2.0.0
 - doctemplates-0.8.1
 - pandoc-citeproc-0.17
 ghc-options:
    "$locals": -fhide-source-paths -Wno-missing-home-modules
-resolver: lts-15.0
+resolver: lts-14.6