.github(workflows), nimble: bump Nim from 1.6.8 to 1.6.10

[ee7]

See the release blog post and changes since 1.6.8.

---

To-do:

• Wait for Nim 1.6.10 release
• Update iffy/install-nim
• Merge .github(workflows): bump iffy/install-nim from 4.4.0 to 4.5.0 #709
• Rebase on main

Previous Nim bump: #666

Command run:

git ls-files -z | xargs -0 sed -i 's/1.6.8/1.6.10/g'  

Double checking:

$ git rev-parse --short HEAD
1a5b35d7
$ git grep --break --heading '1\.6'
.github/workflows/build.yml
42:          version: "binary:1.6.10"

.github/workflows/tests.yml
45:          version: "binary:1.6.10"

configlet.nimble
21:requires "nim >= 1.6.10"