Switch Linux runners to Ubuntu 26.04 except for MIPS (#3703)

The i386 build ran on ubuntu-22.04, which is retired on April 17, 2027,
and ubuntu-24.04 lacks fmod and exp10 in the static i386 libm.

Build on ubuntu-26.04 for every architecture except MIPS, whose cross
toolchains Ubuntu 26.04 no longer ships, and move the decnum, oniguruma
and valgrind workflows too. Most release binaries are now built with
GCC 15 and glibc 2.43, and they remain statically linked.

.github/workflows/decnum.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   disabled:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     steps:
       - name: Clone repository
         uses: actions/checkout@v7