chore: update ubuntu version for ci actions

[imbajin]

test in versions below: (✓ mark as ci passed)

• 18.04
• 20.04 (choose it)
• latest (even pass now, maybe not suitable to use, because cannot control the stable version)

[codecov]

Codecov Report

Merging #1613 (c4fe71b) into master (2a95f9f) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1613      +/-   ##
============================================
+ Coverage     63.27%   63.29%   +0.02%     
- Complexity     6701     6706       +5     
============================================
  Files           418      418              
  Lines         34449    34449              
  Branches       4764     4764              
============================================
+ Hits          21796    21804       +8     
+ Misses        10404    10400       -4     
+ Partials       2249     2245       -4     

Impacted Files	Coverage Δ
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	72.30% <0.00%> (+0.30%)	⬆️
...egraph/backend/store/cassandra/CassandraStore.java	72.64% <0.00%> (+0.85%)	⬆️
...va/com/baidu/hugegraph/task/ServerInfoManager.java	73.59% <0.00%> (+2.24%)	⬆️

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