Use muti-byte string literals for buffer sizes

[Ozaq]

EDIT: I cannot move the PR back to in-draft but this needs to wait until eckit 1.28.6 is released.

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With multi-byte string literals available in eckit we can start using them in fdb.

[ChrisspyB]

This should presumably also update the minimum eckit version in the cmake lists

[Ozaq]

This should presumably also update the minimum eckit version in the cmake lists

Oh good catch, so that should then probably be the next to-be-released version of eckit, right?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 34.78261% with 15 lines in your changes missing coverage. Please review.

Project coverage is 63.84%. Comparing base (d9c77cd) to head (4d96d82).

Files with missing lines	Patch %	Lines
src/fdb5/remote/server/StoreHandler.cc	0.00%	3 Missing ⚠️
src/fdb5/io/LustreSettings.cc	0.00%	2 Missing ⚠️
src/fdb5/remote/server/CatalogueHandler.cc	0.00%	2 Missing ⚠️
src/fdb5/toc/RootManager.cc	33.33%	2 Missing ⚠️
src/fdb5/tools/fdb-hammer.cc	0.00%	2 Missing ⚠️
src/fdb5/database/Manager.cc	0.00%	1 Missing ⚠️
src/fdb5/remote/server/AvailablePortList.cc	0.00%	1 Missing ⚠️
src/fdb5/remote/server/ServerConnection.cc	0.00%	1 Missing ⚠️
src/fdb5/toc/TocStore.cc	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #97   +/-   ##
========================================
  Coverage    63.84%   63.84%           
========================================
  Files          281      281           
  Lines        15815    15815           
  Branches      1643     1643           
========================================
  Hits         10097    10097           
  Misses        5718     5718           

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