[cashtab-components] Replace SLPA references with eToken

Summary:
T2536

Replace SLPA references with eToken

Test Plan:

cd web/cashtab-components
grep -R --exclude-dir={node_modules,dist,coverage,storybook-static} 'SLPA' .

Some remaining variable names in README.md; these will be removed when legacy address formats are removed

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11771