Bump Codecov GitHub action to v3.0.0

[tautschnig]

As of v2.0.2 the recurring issue of uploading errors caused by invalid
tokens should be fixed
(codecov/codecov-action#440). v3.0.0 is the
latest and includes those fixes.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6800 (69ac490) into develop (5a58dff) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6800      +/-   ##
===========================================
- Coverage    76.97%   76.97%   -0.01%     
===========================================
  Files         1593     1593              
  Lines       184107   184104       -3     
===========================================
- Hits        141724   141718       -6     
- Misses       42383    42386       +3     

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_declaration.h	95.23% <ø> (ø)
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
...rc/goto-instrument/contracts/memory_predicates.cpp	83.52% <ø> (-0.57%)	⬇️
src/goto-instrument/contracts/memory_predicates.h	100.00% <ø> (ø)
src/ansi-c/c_typecheck_base.cpp	78.32% <100.00%> (+0.28%)	⬆️
src/goto-instrument/contracts/contracts.cpp	93.82% <100.00%> (ø)
src/linking/remove_internal_symbols.cpp	100.00% <100.00%> (ø)
src/util/expr.cpp	82.83% <0.00%> (-2.24%)	⬇️
src/util/expr.h	97.16% <0.00%> (-0.06%)	⬇️

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