ci: latest pandoc version in gh actions

[sbrunato]

Fixes warning displayed in the Build the docs job in github actions:

/home/runner/work/eodag/eodag/.tox/docs/lib/python3.7/site-packages/nbsphinx/init.py:1058: RuntimeWarning: You are using an unsupported version of pandoc (2.9.2.1).
Your version must be at least (2.14.2) but less than (4.0.0).

[github-actions]

Test Results

       4 files  ±0         4 suites  ±0   3m 57s ⏱️ -26s
   388 tests ±0     386 ✔️ ±0    2 💤 ±0  0 ❌ ±0 
1 552 runs  ±0  1 496 ✔️ ±0  56 💤 ±0  0 ❌ ±0 

Results for commit 72eebcd. ± Comparison against base commit 791fb4d.

[github-actions]

Code Coverage (Ubuntu)

File	Coverage
All files	82%	✅

Minimum allowed coverage is 70%

Generated by 🐒 cobertura-action against 72eebcd

[github-actions]

Code Coverage (Windows)

File	Coverage
All files	77%	✅

Minimum allowed coverage is 70%

Generated by 🐒 cobertura-action against 72eebcd